Yeah, I think that's what "BA" is. Not so sure what it's supposed to prove, I can think of a lot more useful stats. Maybe that's why nobody but Yahoo seems to list it.
The other stats are all attempts to judge how much a player hurts or helps their team.
+/- shows whether or not your player's TEAM was as productive as their opponent. Contrary to popular belief, it has little to do with who a particular player may be facing off against (unless the matchup is so lopsided as to strongly influence the course of the game). It just shows whether your team, or the opposing team was winning when you were in the game. If they send you in for 10 minutes, and during that time your team scores 20 and the opposing team scores 22, your +/- rating for the game is -2.
On court/off court (at which Mikki also lags Justin, and, for that matter, Hawes) also shows whether your team is winning or losing when a particular player's on the floor, but does so per 100 possessions. Like +/-, it reflects the team's success as a whole; if your team is always losing, almost every player on the roster may have a negative score, although it should still help to sort out which players are relatively less crummy. Both of these stats can also be influenced by positional realities; if you are Beno Udrih, you've had a relatively dandy stat all year, because when you were off the floor the team has had no PG at all. So a high rating for Beno doesn't mean that he's necessarily a great player, but it does mean that he's been vital to the team's performance. In positions where proper rotations are possible, on court/off court stats will be influenced by how good or bad the teammates who sub for you are. If you're KG's backup, it will always look bad. If you're splitting time with KT, it'll always look good.
Roland Rating is an attempt to get a more balanced and useful +/- stat. It's sometimes known as "net +/-," and is calculated by team points per 48 when a player's on the court, minus team points per 48 when the player's off court.
In short, all 3 of these stats are ways to try to judge a player's usefulness to his specific team. Because they overlook the usual stuff like points and rebounds, it is possible that a truly stunning help defender, who never made an assist, rebound, steal, block or point, could yet have positive scores on all of them. It's also normal for a player who puts up big numbers, but who hogs the ball, shows poor judgment, and generally hurts their team to have a very negative rating. This distinguishes these stats from older ones, like PER, which are 90% offensive, do not show any "intangibles," and do not show whether, overall, you're helping your team or making it worse.