BU is back up or what's left over from Round 1 bidding - could also be called Round 2 and when you go into FLICA it is in fact called just that.

The chart indicates the date of hire of the last person able to hold that position in line.

It wouldn't be accurate but you could kind of interpolate between the most junior R1 First Officer and the most junior SCR Captain and kind of guess how big the R1 pool is.