There are three forms of "dead' when it comes to batteries.
But first, let's talk about what's in that "battery." It is a collection of cells, and in the case of a 12V battery in your car, there are six of them connected in series. This means that all of the current that comes out of one terminal and goes back into the other must flow through all six cells. This is important in the context of behavior when a battery fails. Now to the three forms of failure:
1. An effectively-open cell. This is a cell that may show it's normal 2V were you to check it with a voltmeter when there's no load or charge on it, but as soon as you try to draw any sort of current out of it, or charge it, the amperage that can flow through it is basically zero. Note that if you have a battery that can deliver 600 amps to crank the car all six cells must be able to pass 600 amps. If just one can only pass 20 amps then only 20 amps come out of the battery, period. The battery can only pass as much current as the least capable cell can deliver.
2. An effectively-shorted cell. This is a cell that shows an effective zero volts when not under load or being charged. This cell behaves very differently. It will allow virtually unlimited current to flow through it but contributes nothing on its own. If charged it will either turn most of the energy that flows through it to heat or simply pass it through without doing anything.
3. An actually open cell. The battery shows 0V across the terminals because it is literally an open circuit somewhere inside the case. This is not a common failure but it does happen. In this case you open the door and get no lights, your fob doesn't work to push to unlock, etc.
How and whether you can jump the car depends on which failure you have if you have a failure.
If the battery is just discharged when you try to jump it the car you connect it to looks like a big and very fast charger. This is why you never, ever make the last connection to the negative post -- always to the block or other big honking piece of metal well away from the battery. Batteries always produce hydrogen when charged or discharged and if it is discharged but otherwise ok, and in a few of the above cases, you will produce a big spark and if there is hydrogen there you can get a "kaboom", blowing up the battery and spraying acid all over everything, including you.
Now if it's just discharged then you have two things going on when trying to start: The battery is being charged and drawing a lot of current down the cables to do that (this can be several hundred amps as the other battery is essentially an unregulated, "as much as you got" current source limited only by differential voltage and the internal resistance of the discharged battery) while at the same time whatever has been delivered into it can also help turn the starting motor. This is why when you jump you usually want to raise the RPM of the boosting vehicle after connecting the cables as you want the alternator in that car delivering whatever it can, and it's also why you want nice, thick, low-resistance cables. In this case you want to run that high idle for a couple of minutes before attempting to start because once the voltage on the discharged battery starts to come up the draw to charge it will go down somewhat. The odds of success in this situation are extremely high as the actual current that has to come down the cables to start is a small fraction, perhaps 50 or 100 amps, since most of the current can and does come from the local battery in the jumped vehicle.
If you have situation #1 or #3 then the battery in the car that is dead is contributing an effective ZERO to the jump. Now it is all about one thing: Can you deliver enough current down the jumper cables alone to turn the starter? If yes, it starts, if no, it doesn't. This is the reason you buy big and beefy jumper cables; the smaller, lighter, easier to carry ones have no prayer in Hell of being able to deliver 300, 400 or 500+ amps down them. No way, no how and I don''t care if they're called "heavy duty" or not. Note that in situation #1 a restart after a jump almost-never works since the battery is compromised and can't deliver material amounts of current and in situation #3 you may destroy the alternator in the jumped vehicle as soon as you disconnect the cables because of back-EMF from the alternator itself; it is the low-resistance battery that buffers back-EMF normally and the regulator cannot respond fast enough to prevent this from happening. The bad news is that if you have a true open circuit and it happened when you were driving you may have already compromised or destroyed either the regulator or rectifier in the alternator, so be prepared to change it out once you get a new battery.
If you have situation #2 you're fooked. The jump will probably fail because in addition to having to supply all of the starting current down the jumper cables in addition you have a parasitic drain in that the jumping vehicle will be trying to provide "charge" current to a cell that is shorted. In other words not only do you have to provide ALL of the current to turn the starter there's even more drain that you must be able to provide too! Unless you have cables with wire the size of your finger in diameter (and you don't) the odds of success are very low.
I've seen all of these over the years. #2 and #3 type failures usually happen without warning and #2s are the nastiest sort. You're driving, everything seems ok, you get out of the car, shut down, go in a store or something and a few minutes later come out, get back in and.... no start. My kid had this one happen to her about a year ago; she called, I drove over with my truck (and a nice beefy set of cables) and could not get enough amps down them to jump it. The shorted cell was pulling enough that even with a nice big fat truck and its alternator you just couldn't get enough amps down the wire to be effectively the sole current source to turn the engine over. We wound up pulling the battery, taking it over to WallyWorld and buying other one; put that one in and it fired right up.
If you do get warning (the engine rolls over slower than normal, and you note it) change the battery now. This is a warning of an impending Type #1 failure.
In short jumps almost always work if the battery is just flat due to leaving the lights on or something like that. But if the battery itself is compromised the odds go way down because you now have to source all of the cranking amps down those jumper wires and they're just not that large compared with the big fat primary cable and ground strap that are in the car itself, never mind that they're also a lot longer and thus there's more resistance in them and more voltage and current drop than in the primary wiring in the vehicle itself.
If you happen to have a set of cables made out of 4/0 welding cable, well, then none of this applies because in typical lengths for a jumper cable now you can realistically draw 300+ amps down it, provided your clamps have enough area to transfer the power -- but there is nobody I'm aware of that sells those "in a box" at the local store. And no, buying the thicker ones if they're also longer doesn't help a bit! For example there are supposed "800A" cables on the market that are #1 wire but 25' long. That's complete and utter BS; there is no way you can get a usable voltage through those cables at 800A.
I mean, look -- you see these so-called "super heavy duty" alleged "500 amp" jumper cables and they're six gauge. Ignore the BS about ampacity as that's a function of how hot it will get and when it melts the insulation. What you want is voltage drop at a given amperage. A pair of 6 ga jumper cables 12' in length (common "heavy duty" cables) called up on to deliver 500 amps will result in 7.26V being delivered. If you try to source the entire starter current off the cables you will fail.
Minimum start voltage for most vehicles is in the range of 10.5V (below that and the ECU refuses to provide fuel as minimum cranking speed is not achieved) and the most you can get out of those cables and remain within that boundary is ~150A. Any more than that when you hit the key and the car does not start. BTW for those so-called "800A RV and big truck" cables? At a mere 250 amps (about HALF of what it takes to roll over a decent-sized diesel in a truck or RV) you hit that 10.5V threshold. In other words if you have a shorted cell even those probably won't start your car because they're too long! Now if they were 12' long instead of 25' you could get the whole 500A you need down them but they're not.
Incidentally if you try to draw 800A down those alleged "800A" cables the voltage on the other end will be 7V. If you try to do that for any length of time you'll also light them on fire.
BTW those "emergency jumper packs" are flat-out worthless for anything other than a discharged battery. The cable size on them has no prayer in Hell of delivering the claimed current and, for that matter, neither can the battery in them. The only purpose they have is to get enough charge into a properly functioning but otherwise discharged battery so you can source the starting current out of the car's battery, not the "jumper pack's" one. For any actual battery failure they're flat-out worthless.