Local Cap Rates for Multi Units

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I'm a wholesaler here in Phx.  I'm looking into marketing for multi unit properties; duplex, triplex, quad and up to say 5 units. I've been researching cap rates for the Phoenix area multi units based on a May 2013 national report I found which broke cap rates down to city level and classes.

My question has to do with how to they distinguish between Class A, B and C when breaking out cap rates. A having the lowest cap rate, C the highest. Can't seem to figure it out why the distinction.

Also, in talking to different investors here, it seems like the cap rate (I assume A, B,C) depends on what area of town.. 

Can someone explain where the classifications come from?

Further I assume then when analyzing a potential multi unit deal, say a 4 plex, that I should compute potential purchase price on the expected cap rate (A, B, C) using a 50% rule as a first pass?

Thanks in advance for any guidance provided!