sendmail.cf R and S Commands
In the sendmail.cf file, the rulesets are defined using commands coded as S n, where n specifies the ruleset that is considered the current one.
The rules themselves appear in commands coded as R. As each R command is read, it is added to the current ruleset.
If you're dealing only with the sendmail.mc file, you won't need to worry about S commands at all, as the macros will build those for you. You will need to manually code your R rules.
A sendmail ruleset therefore looks like:
S n
R lhs rhs
R lhs2 rhs2