[Mp4-tech] Difficulties understanding MPEG-4 audio profiles and levels

[Marc Jaeckle]

I'm having difficulties to understand which sampling rates and bit 
rates are allowed for a given profile @ level combination. The standard 
only seems to specify the maximum Processor Complexity Units (PCUs) and 
RAM Complexity Units (RCUs) for every level, the PCU and RCU number of 
every object type at 48kHz (in the case of AAC) and the types of 
objects allowed for a profile.
What I don't understand is how the sampling rate and bit rate are 
related to the complexity (measured in RCU and PCU). For some object 
types the PCU seems to be proportional to the sampling rate but what 
about the others (e.g. CELP object types) and what about the RCU?
... and why are some levels defined by sampling rate while others are 
defined by complexity or the number of objects.
Any help would be greatly appreciated :-)
Gruß
Marc