I think the Op is referring to 500 Multi PIC.
As Pilotchute says about low capacity AOC holders needing a pilot to have 500 hours multi command for upgrade, so they then ask for it of the FO upon entry in case of a quick stepup, or because they stupidly don't know any better. ICUS time is also acceptable and can be gained while in that right seat, however let's not get into that particular can of worms here as thats a whole new thread on its own as has been discussed before, and there have been other threads about this frustrating hours blockade in the past too!
Not to be confused with 500hrs on Type for heavy turboprops and jets for ease of transfer of the type between different countries licences.