What is an RSA subkey? [eom]

[Robert J. Hansen]

We're going to need a lot more info than this.

"What is an RSA subkey?"  Well, it's a part of a certificate which
provides the capability to do some or all of signing, certifying,
authenticating, and/or encryption, using the RSA algorithm to provide
its cryptographic needs.

If that answer is helpful to you, you're quite welcome.  :)  But if it's
not, you really need to drill down and tell us what precisely you're
looking to learn.