If full throttle is required to give a certain manifold pressure at any selected r.p.m., then the altitude at which this occurs is the full-throttle height for that condition of r.p.m. and boost.
I found that from flitting around on google, and also this:
Spitfire IX Trials at +25 boost
I believe the term only applies to supercharged engines, but may well be wrong. Hope that helps until someone with a better understanding comes along!
Regards,
Spr K, (retd)
PS: I am NOT a test pilot, or involved with testing... Just trying to help