@Jay Ben, the class rating system is generally subjective.
For properties: things like age and amenities drive the rating. 'A' is relatively newly built with a full amenity package: typically resort level pool, fitness center, dog park, in unit washer and dryer, playgound(s) if mostly family area, maybe covered parking/garages, etc. Class B would be older (maybe 10+ yrs old) with the same amenities, or you could argue that new construction, but with cheap finishes and "bare bones" amenities, would also be B.
For area: it commonly comes down to crime, schools, and convenience/entertainment access. Good schools and low crime, near interstate access but 15-20 min drive to major employment hub may be a Class B area.
Again, there is no definitive way to break this down. I seen deals marketed as Class B properties that I would certainly classify as C-ish, and yet have also described things as B, that others say "that's an A or A-". I like walkable areas, so anything that requires full car use is Class B to C, to me, but others don't mind driving 20+ minutes for everything, so they think of those areas as A.
@Ian Davies, to echo Chris: I would be very cautious of "high" IRR deals in today's market. Personally, I perceive a fair amount of risk in the market. And if a group is out there projecting that 20% IRRs are still achievable for a typical value-add play, I get very suspicious, and ask for a lot of "projections vs actuals" on current deals to confirm their assumptions.