I imagine that would be when a bowler is able to use his preferred equipment, on his preferred line with his preferred release and hand position and speed.
Brad
The only thing I might add to this is decent carry.
I know I have times that I can get to the pocket easily with my "A" game but my carry is suspect. In those cases I often start to play with speed and release to try and increase carry.
I would not want a steady diet of my "A" game. It wouldn't prepare me for when conditions where I don't match up so well, plus it would inflate your average.
I would say about 10% of the time to use your "A" game, 10% of the time bowling on your nightmare condition, with the rest being various conditions in the middle somewhere.
"A" games generally fit a persons body type and personality. Only a few of the most gifted athletes can alter their game and make it as good as their "A" game.
In fact look at the tour, even most of the top bowers have an "A" game and they can win when that shot is present. The top stars have alternate games good enough to get them in contention on other shots.
But most try to use equipment and lane play to get them in their comfort zone.