SFTRUNCATEy commands removes part of the current, i.e. active, i.e. currently active, soundfile. It will return the number of remaining samples.
Note that the segment directory will not be updated.
var := SFTRUNCATE endaddress
This will truncate the current soundfile at the address endaddress. This document doesn't tell us how this address is specified - my guess is that if works like a segment expression, but don't rely on that.
var := SFTRUNCATE segment
This will remove everything from the current soundfile before and after the respective segment. This document doesn't tell us if segment has to be the name of a segment or if you may use a full segment expression. My guess is the latter, but don't rely on that.