https://www.gnu.org/software/emacs/manual/html_node/sasl/Steps.html
3.3 Steps
A step (
sasl-step
object) is an abstraction of authentication “step” which holds the response value and the next entry point for the authentication process (the latter is not accessible).
Looks like it’s for if you’re using an emacs IRC client or mail client or XMPP client, to do part of the authentication.