The trim is just the difference between the inducer and exducer of the compressor or turbine wheels them self's. The A/R of the turbo, wich MOST people refer to theres turbos "OH, hey... check out my T70 turbo" or "im runing a 60ar turbo...im cool". But really, the AR plays a small role in the turbos job. It does have its role though, but the main ingredient in the turbo, are the actual compressor and turbine blades... (explaining the AR will take a while)
I always refer to my turbo by the size of the exducer (or larger part of the blades) on the compressor (cold side).
**The inducer and exducer are reversed on the turbine (hot side). So the larger diameter is called the inducer.