Would it also be correct to say "Ele explicou seu modelo." ("He explained his model.")? Is the "o" necessary in this case? (Ref: "Ele explicou o seu modelo." is the example posted by Duo.)
It might help clarify your thoughts and his ownership of the model, but it's not strictly necessary, no. Furthermore, due to "seu" being able to stand for "de você" e "dela" (in context, of course), both "your model" and "her model" are also accepted as correct answers in this case.