Apparently we may have skipped a new bit of terminology in the swim plan lexicon! "B" is for Build, which means each repetition is a little faster than the last. So build is negative splitting across the whole set.
For example 4 x 50B on 0:15E is;
- swim first 50 in 1:00 then take 15 sec rest
- swim second 50 in 0:58 then take 15 sec rest
- swim third 50 in 0:56 min then take 15 sec rest
- swim fourth 50 in 0:55 or faster then take 15 sec rest