Please contact your processor for the most up to date AVS code matrix. 

All Card Types: 

Y - Match (street address and 5-digit zip both match)
A - Partial Match (street address, but not zip code)
Z - Partial Match (5-digit zip code matches, but not street address)
N - No Match (neither zip nor address match)
U - System Unavailable 

U can mean the information wasn't available, or that data is a non US address, or that the bank couldn't verify.  

Visa Specific:

E - AVS data is invalid

MasterCard Specific: 

X - Match (street address and 9-digit zip match)

MasterCard & Discover: 

W - Partial Match (9-digit zip is a match, but street address does not) 

Visa, MasterCard and AMEX

R - System unavailable, retry if possible