As we know, we have monoid objects (or ring objects) in monoidal categories. For example, the monoid objects in $\mathrm{Vect}\; k$ are $k-$algebras.
I want to question that can we define a "subring object" to categorification the notion of "subalgebra"?
It seems hard to have the notion "sub" in categories. And I searched the term "relative category", but it is a little hard to understand and seems to have no relation to us.