The best way to differentiate members would be:
Core Member: Those are the specific members you actually want in a standard team, this usually can work among different leaders of the same attribute or race. The most common Core members tend to be “The Witches” and “Protagonists” because they can be used in most mono attribute teams.
Add-on Member: Usually cards that work pretty well with a specific leader, and they tend to provide specific team skills or combine skills, but might not do very well when used without the required leader. Some examples are Nohime (Nobunaga), Meng Huo (Zhurong), Lady Whitebones (Daoloth), Phi (Li Tieguai and Turing), and Epsilon+Delta..
Filler Member: A general name for members that can be used in a team just because you can’t come up with anything better to put in that slot. In most cases you will see players adding over the top damage boosters.
Read next: Team Skills, Leader Skills & Active Skills