In all those books I listed, and more, e.g., on axiomatic set theory and other foundations, never saw definition or mention of codomain. So, the term is obscure. Readers are supposed to guess at the meaning of obscure terms?

What I did was follow, as in the references, long established convention, that for a function to be a vector at least it had to be in a vector space where (1) can multiply a function by a number (e.g., reals or complex) and (2) add two functions and still get a function in the vector space. To be general, I omitted metrics, inner products, topologies, convergence, probability spaces, and more.

Or, as in the references I gave, math talks about vector spaces and vectors, and each vector is in a vector space. The references are awash in definitions of vector spaces with (1) and (2) and much more.

Computing is awash in indexes for data, e.g., B-trees, SQL (structured query language) operations on relational data bases, addressing in central processors, collection classes in Microsoft's .NET, REDIS, and calling all such also functions confuses established material, conventions, and understanding.