How to make a Mask Texture for a Physical Material Mask ?

Hey everyone!
I’m having an issue with my Physical Material Mask. I’m trying to input a mask texture here


but it won’t let me add anything. I’ve seen on previously asked question that you had to have a mask map with 8 color (RGB-CMY-WB) but what does that mean ? I’m making a test texture map in photoshop, what would that stand for, do I need to have 8 channels? or 8 layers set as screen or something like that ?

There’s not much on the subject, most of the links I found are either deleted or not what I’m looking for. And I can’t seem to find any Documentation on Unreal for that either ! I’ve seen someone mention Blender but I’ve never used it for that purpose and I’m not really an expert so I don’t know how to make an RGB PNG on Blender…

Please help !! :folded_hands: