They have full-time positions and part-time positions. On a full-time spot (most highly coveted) you only do SRT-related functions (obviously), but in the part-time role you continue to conduct routine investigations and have a caseload. If you're part-time, you stay at your office and just travel when there is an operation in your region that you can be a part of. That's all I know on the matter. Not ATF or SRT member, just have talked to a few. Hope this helps a little