There isn't a lot information on TORSOCKS_ISOLATE_PID yet. Will be introduced in the torsocks version which is now in Debian version 9 codename Stretch.
- IsolatePID is a new option that will make torsocks set the SOCKS5 username and password automatically to provide isolation on Tor side. You can use this with the -i,--isolate command added or TORSOCKS_ISOLATE_PID env. variable.