HELP! i cant distinguish these two 1.volume absorption and RGB absorption

In my recall, volume absorption is like how much light passing through the object

But recently i watched a ‘BLender cookie’ video about a node group like RGB absorption,
is also about how much light passing through object.

Are these two the same?

May anyone help me out on this pls~? or even maybe i misunderstand the volume absorption as well.
Cuz i think people plug in volume absorption to [volume]
But plug in RGB absorption to [surface]

I am so confused, anyone can clear my thought pls~ :(?

I am so desperated.