SFTRUNCATEy commands removes part of the current, i.e. active, i.e. currently active, soundfile. It will return the number of remaining samples.
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.