This is copy/paste from some German gun forum, translated from german via Google translate:
"The designation +P and +P+ are pure US marketing. In EU there is no such thing and CIP knows only a permissible maximum gas pressure, with .38 Spacial 1500 bar. Any EU-approved ammo must be within this CIP limit.
It's a bit different in the USA. Roughly applies
- without additional name = standard gas pressure, at .38 Spl 17000PSI / 1172 bar
- with +P as name depending on caliber 5 to 10% above. In the case of the 38 Spl, that's 18500 psi / 1275 bar
- If there is still +P+, again 5 to 10% above +P pressure
SAAMI also measures things differently.
Basically, for us in the EU, it can be said that +P and +P+ cartridges will make more use of the allowable gas pressure limit without exceeding the CIP limits."