Poor choice of words on my part. You can log anything you want, including time in coach class commuting to/from work.

But to count for FAA ratings and currency, sim training has to be conducted by an authorized instructor who has to sign the record. The record could be your airline's training form, as opposed to your logbook.

I would also venture a guess that most employers would not accept sim time without an instructor signature.

You can't just go knock out 6 approaches, a hold, and intercepting/tracking in a frasca all by yourself, log it, and be legal.

If I want to brush up on my skills after extended mil leave, I just use MS Flight Sim. Cheaper than a Frasca or ATC and gets the job done for scan and flows. But I don't log it.

Also regarding 121 sim instructors signing your logbook...some are not CFI's so they cannot sign part 91 endorsements like BFR or IPC. They are 121 instructors and will probably only feel comfortable signing 121 paperwork.