packaging but this explains it better.
copied from acam on calguns
XM193= 5.56, 20 round brown box, 500rnds per case.
XM193BK= 5.56, Loose Bulk, 1000 rnds per case. Not as pretty as the boxed stuff where some rounds may have small dents but still goes boom.
XM193A and XM193AF= 5.56, rounds packed in ten round stripper clips, 900 per case.
XM193C= 5.56 commercial package, American Eagle Tactical "Black Box," 20 round box.
XM193CBP= 5.56, commercial package, American Eagle Tactical "Black Box", Loose rounds in 200 round bulk (value) pack.
The letters usually designate the packaging. Some like the brown box stuff more that the black box AE. Look at the reviews and shoot what your rifle likes. Hope that helps.
Also From online vendor..
"Federal announces that packaging for XM193 is now done in Anoka, so a new part number (XM193F) was needed for FET purposes. THIS IS THE SAME ammo as XM193, and XM193C and apparently replaces both of these items."