i wondered the same thing a while back with rebug, didnt get an answer, dg'd anyway and was fine, i'm assuming its checked by default or something, as i said, i ignored it and i was fine, but can't say for sure what it is
it means that the spkg (a module responsible for installing firmware updates) has been patched so it allows CFW to be installed on this FW that is spkg patched. Normally, a OFW 4.30 had blacklisted 3.55 HMAC Keys, once patched it will remove blacklisted 3.55 HMAC.
If this spkg is not patched on these new CFW like 4.21 REX or 4.30 Rogero, there is no way you can downgrade back to 3.55 without a flasher. But yes, they patch spkg so you can just go back to 3.55 whenever you want without the need of a flasher.