Relationship between special RSA modulus and quadratic residue in CL (Camenisch-Lysyanskaya) signature

Question

I'm studying the CL(Camenisch-Lysyanskaya) signature ("A signature scheme with efficient protocol proposed by Camenisch-Lysyanskaya").

However, I cannot understand the relationship between special RSA modulus and quadratic residue.

What is relationship between Special RSA modulus and quadratic residue?

and please recommend the references about special rsa modulus.

Thank you for your help.