The "A Mode" refers to the normal or preferred mode of operation. By contrast, the "B Mode" is a sort of graceful failure mode that kicks in when something such as an animatronic figure is not working properly; rather than shut down the entire ride, Disney plays an alternate, substituted sequence that does not use the failed component. This video illustrates the difference between the "A Mode" and "B Mode" in the Kylo Ren scene where he demands that the recruits tell him the location of the secret base.
The "B Mode" footage shown here occurred during my New Year's Eve live stream and I talked about it on the stream after the ride at 51:31 ruclips.net/user/liveyLW6esGlyGE?si=L293S9qPp512uKL...