This discussion is interesting and I can't help but join. I am only a social ballroom dancer, mostly LATIN, and don't compete, however, I had taken several lessons from famous dance instructors, former champions and adjudicators like Ron Montez, Corky Ballas, The late Chris Morris to name a few. I also studied some of their tapes.
I learned to use a1, a2 when doing the corta jaca. The man starts with his right foot when dancing the corta jaca in the promenade position (someone mentioned this in this discussion), lady on his right side. He starts with his left foot when doing it facing his partner. Left foot forward with weight on the heel followed with right foot forward with a simultaneous transfer of weight to the right foot( a1 ). Left foot back with weight on his left toe followed with right foot back with a simultaneous transfer of weight to the right foot ( a2 ). Repeat as you wish.