@iftachzi to troubleshoot (and develop) session relevance, it is helpful to use something like the Presentation Debugger, or Session Relevance Tester rather than directly via the REST API.
If using Presentation Debugger with your above session relevance, you’ll receive the following error:
In this case, the reason for the error is due to the fact that
microsoft is not an operator. Since the source property of a
bes fixlet object returns a string, you’ll have to surround
microsoft with double quotes. Additionally, note that string comparisons within BigFix are case sensitive, so, you’ll want something like:
(id of it, name of it) of bes fixlets whose ((name of it does not contain "Microsoft" and name of it does not contain "Superseded" and source of it as lowercase = "microsoft") and applicable computer count of it > 0)