Having completed my discussion of operatic singer types, I have begun to wonder if there is anything I left out. Coloratura bass, not unknown in the Baroque era, is one. Baroque coloratura was applied indiscriminately to virtually everything.
And there is this mysterious German term: Zwischenfach. It translates "between category." This is hard to describe. People write dissertations on this. I notice in the German language version of Wikipedia that it has no entry of its own but comes up in entries for both individual male and female singers.
In German there is a very detailed and specific set of categories, and then everything is squeezed into it. Anything that crosses into two categories is a Zwischenfach. Two examples are Kundry in Parsifal and Venus in Tannhäuser. These roles are basically mezzos with high extensions. It's possible that the category bass-baritone is itself a Zwischenfach.
I'm going to tell my opinion now. It is nice to imagine that things fall into tidy discreet categories the way they do in the German Fach system. In reality they don't. I like, for instance, to claim that Maria Callas is a mezzo, and this is the explanation for her preeminence in the role of Norma. You see, Norma is also a mezzo. She destroyed her voice by pushing it too high with too much weight. I said. But perhaps she was just a Zwischenfach.
Life is not tidy. Some composers may have been aware of these categories. Verdi seems to have started out with only a vague idea of what was physically possible for a singer, but then developed into a wise and very skillful composer for different types of voices. Wagner seems simply not to have cared. He composed whatever he wanted, and left it to management to find someone suitable. His dramatic tenor was phenomenal. Pity subsequent generations who try to follow in his footsteps.
From the point of view of the individual singer it is most important to understand the weight of the voice required for a role and how ones own voice might manage it. You cannot make your voice heavier than it is.
It's ok if you ignore Zwischenfach.