SAMMI pressure for 9mm is 35,000 psi.
SAMMI pressure for +P 9mm is 38,500 psi.
NATO pressure for 9mm is 42,000 psi.
Thre is no specification for +P+, but it should be higher than +P, if there is logic in cartrige nomenclature. You know that isn't always true, though, so +P+ may, in some cases, be more marketing than actual description.
Also, pressure and velocity aren't always totally linked - sometimes pressure in a faster load can be lower, depending on bullet, primer, powder, etc.
These are maximum pressures, too, so any given loading may be under that pressure level. For example, Winchester White Box 9mm is generally loaded to a lower level than most other 9mm. I often shoot Blazer 9mm in my SIG, but bought some PMC ammo the other day, and it showed the signs of lower slide velocity (stovepipe jams), leading me to believe it was loaded to significantly lower pressure than maximum.