Modular equivalence in Agrawal's conjecture

Question

In Agrawal's conjecture:

If $r$ is a prime number that does not divide $n$ and if $(X-1)^n ≡ X^n-1 \pmod{X^r-1, n}$, then either $n$ is prime or $n^2 ≡ 1\pmod r$.

How to understand $\pmod{X^r-1, n}$?

I searched for "modular equivalence" but did not find an answer for this notation (with comma in parentheses).

Answer 1

I think it's simply remainder left after dividing $X^{r}-1$ by $n$.