Hey there @me-zo! Each of the FAB licenses for content have their own rules. UEFN assets themselves will remain UEFN only in most cases unless on FAB, then they will go by the respective license attached. For example, many will share the “UEFN - Reference Only” license which would only allow you to use them in UEFN as reference.
I would definitely take a look at which assets on FAB you’re considering and verify which licenses are available. Personal licenses that aren’t reference can be used commercially, but what determines personal or business is how much your work makes. I’d recommend taking a thorough read through the FAB EULA before making any decisions asset wise.