In SmileBasic V1,
BGMSTOP takes no parameters. It stops whatever BGM sound is playing, immediately.
BGMSTOP will stop music playing on any of the 8 tracks.
BGMSTOP is used without any parameters, the music on track 0 is stopped immediately.
BGMSTOP is used with one parameter, it is used as the track number (0 to 7). The music playing on that track is stopped immediately.
BGMSTOP is used with two paramaters, the first is the track number, and the second is the fade time. The music playing on the specified track gets gradually quieter, disappearing after the specified time (measured in seconds). Unlike most parameters to most commands, the fade time is not rounded down to an integer.
See also BGMCHK (Function).
No information on SmileBasic V3.