/X (/X means you have literally no navigation capabilities or transponder.)
Here is a simple chart with them (taken from the VATSIM website).
If you are flying a PMDG, iFly, CaptainSim, etc, that has an FMC, you are most likely a /L.
Default Flight Simulator GPS? most likely a /W, as you don’t really have RNAV and the GPS is so out of date its almost like no GPS.
Just a classic 6 pack in a 172? probably a /A.
Also, a 737 is not a “Heavy” so do not file H/B737. And there is no more /Q. If you are still filing /Q, it is /L now.