If the parts are shown in the order A B B1 A1, I guess that the tune is played in that order, and B1 is a variation of B and A1 is a variation of A. But I also think several players would learn all parts with variations and play them in arbritary order. Try to listen to a recoding to find out.
Sometimes the composer will provide the playing order. Sometimes a part will a repeat sign and have different endings (like the last measure) which suggests to me you play (like B1) once with the first ending and repeat using the second ending. Then move on to another measure Like Jan, I'd suggest a recording.
Nickcd I checked the tab on the HO (Jan's tab BTW) and it is played AA, BB.....etc. There are a couple of old threads on this tune, and one includes a recording of someone playing the tune from Krassen's tab.