category theory - Meaning of ? applied to morphisms

Question

I think I understand what ? means when applied to objects (such as M ? M), but what does it mean when applied to morphisms (such as η?1). Is it just composition?

question from:https://stackoverflow.com/questions/65894395/meaning-of-%e2%8a%97-applied-to-morphisms

Answer 1

Assuming the question is about monoidal categories (for which https://math.stackexchange.com/ is the more appropriate site), ? : C x C -> C is a functor, and therefore has an action on objects and on morphisms. Given objects X, X', Y, Y' in C and morphisms x : X -> X' and y : Y -> Y', x ? y : X ? X' -> Y ? Y' is a morphism in C.