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RMA vs ARM Maps?

Hi people , i got a question about RMA and ARM maps
i know that ARM and RMA maps are both a channel packed map that can be created using photoshop
substance etc but i’m wondering whats the difference between the two of them?
while Red channel is Roughness for RMA and Red channel is AO for ARM, whats the actual difference in game industry pipeline?
thank you for reading !

In general there is no practical difference. You can use any combination you want, MRA, RMA, MAR, whatever.

From a more technical standpoint if you are using DXT1 compression for your masks texture then the green channel will have higher bit depth than the red and blue channels (6 bits vs. 5) so it is probably best to put the roughness into the green channel since it will usually be the highest detail (and in the most confined value range) of the three maps.

In practice, I find the difference is fairly negligible, but that’s more a matter of opinion.

Another good option is if you don’t need an AO or metallic map (or can get away with a reduced resolution on them), then you can store your roughess in the base color’s alpha channel as the alpha is 8 bits. This is what Epic sometimes does (Kite demo assets for example). However using the alpha has the cost of adding a whole additional texture so keep that in mind.