Each of the 4 letters has a different meaning:
1st: The media type
S = CD/DVD
U = UMD
B = BluRay
2nd: Licensing
C = 1st party (Sony)
L = 3rd party
3rd: Region
E = Europe/PAL
J/P = Asia/Japan
U = US/Canada
4th: Release
D = Demo
M = Malasian Release
S = Retail Release