What does it mean to define a norm as $\|.\|_{A, B}$? (e.g. $\|.\|_{\infty, [-1, 1]}$)

Question

I have seen when many instances of $\|.\|_{A}$, e.g. $A = \infty$ or $1$, etc. What does it when we write a norm as $\|.\|_{A,B}$?

For example see theorem 3 of this, which uses $\|.\|_{\infty, [-1, 1]}$.

Answer 1

From your link: see the paragraph starting with Notation: on page 3.