Unified burst disks is a 1 piece safety device for paintball tanks (air or CO2) which prevent them form becoming over pressurized.  They replaced “burst disks” which were a 2 peice system.  We will still refer to the newer unified burst disk as a burst disk.  Co2 tanks have 2 burst disks, N2 tanks will have 2 burst disks.

The burst disk markings can very by manufacturer.  Usually they are 1.8k, 3K, 5k, 7k.  Some manufacturers will mark “operating pressure” on them while others will mark burst pressure.  I belive there is talk now to standardize this.  1.8k=3k, because the 1800 psi pressure marking is operating pressure verses the 3k burst pressure. The 3k is the max pressure it will burst at.  I could burst anywhere from 2200PSI-3000 PSI.  On a normal day outside, a co2 will output 850PSI.
CO2 tanks will have a 1.8k or 3k marking burst disk.

N2 tanks have 2 disks.  1 To prevent the pressure vessel itself (tank) from bring over pressured, the other to protect the gun in case of a regulator failure.  The tank burst disk will be a higher rating, for a 3000PSI tank it will be 5k marking, for a 4500PSI tank it will be a 7k marking.

The n2 regulators burst disk will be the same as a CO2 burst disk marked at 1.8k or 3k.

Of course if you have questions call your tanks manufacturer before you do anything.
When in doubt always replace your burst disk with one of the same marking and never use a higher rated burst disk when you should be using a lower rated one. Thats how accident happen and it could seriously hurt or kill someone.

If you have to replace a burst disk, you do not have to seal the treads with teflon tape or thread sealant.  It will just screw in hand tight with a wrench.  If it leaks a little give it a little extra nudge.  Do not over tighten it.