If it's the same model of reel, e.g. an Exage. the F indicates that it's front drag. The following A, B, C, etc indicates the sequential series. The FA is an older model than an FB or FC.
The original Baitrunner ST FA model (up to the 2014 catalogue) of small baitrunner has now been updated to a new ST FB version in the current 2015 catalogue. The differences are minor, and mainly cosmetic. I can see that the handle and roller are different and they've taken a gold band off the spool. Internal specs look the same.