The Atari QL-Emulators come with the additional toolkits, ATARI_REXT and ATARIDOS.
This command is used to enable various commands in the ATARI_REXT toolkit as well as the sound extensions (such as BELL).
It therefore replaced the original SND_EXT command.
ATARI_REXT pre v2.37 may crash SMS.
See also SND_EXT.