whilst there is no limit on how you create your packed textures certain engines require your input textures to conform to specific channels, and your inputs 

need to be at the Power of 2 i.e 512x512, 1024x1024, 2048x2048, 4096x4096 etc.... 


GameGuruMAX uses a Surface Map:


RED Channel = Ambient Occlusion

GREEN Channel = Gloss

BLUE Channel = Metalness

ALPHA Channel = NOT USED




Unity uses a Metallic-Smoothness Map:


RED Channel = Metallic

GREEN Channel = NOT USED

BLUE Channel = NOT USED

ALPHA Channel = Smoothness




Unreal uses a Metallic-Roughness Map:


RED Channel = Ambient Occlusion

GREEN Channel = Roughness

BLUE Channel = Metalness

ALPHA Channel = NOT USED