asus merlin firmware upgrade

When I load up Merlin 386.7, is this an "add-on" to Asus firmware or "replacement". I am asking because I see Asus updating firmware for my router. If I upload Merlin firmware, then there is no need for Asus updated firmware, correct? I just upload stable Merlin releases?

Hi I own the said router and have used/updated it successfully for quite a few years now. I am aware that Asus have recently (past 12-24 months) been adding some check in the update that prevents unofficial firmware from being upgraded; requiring some more technical skills to overcome. However...