Definition of quotient category

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Is there any reason why only gluing of morphisms sharing domain and codomain is usually allowed in the definition of quotient category?

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So you want to identify $f : A \to B$ with $f' : A' \to B'$ even if $A,A'$ and $B,B'$ have nothing to do with each other? But this means that at least in the quotient we will need isomorphisms $A \cong A'$ and $B \cong B'$ such that $f,f'$ get identified along these. But adding some new morphisms between given objects is not the task of quotients. This is a different type of free construction.