B-roll
Linear editing systems cannot dissolve between clips on the same video tape. Hence, one of these clips will need to be dubbed onto a new video tape. EDLs designate these occurrences by marking such dissolves' source reels as B-roll of "b-reels". For example, the EDL will change the 8th character of the reel name to the letter B.
However, sometimes editors will (confusingly) use the letter B to designate time code breaks on a video tape. If there is broken time code on a video tape, there will be two (or more) instances of a particular time code on the video tape. When re-capturing, it can be ambiguous as to which timecode is the right one. The letter B may indicate that the right time code is from the second set of timecode on the video tape.
Read more about this topic: Edit Decision List