A rule to match percent hacks
If the MTA tries to rewrite an address of the form A%B and fails, it tries one extra rule before falling through and treating this address form as A%B@localhost. This extra rule is the percent hack rule. The pattern is $%. The pattern never changes. This rule is only activated when a local part containing a percent sign has failed to rewrite any other way (including the match-all rule described below).
The percent hack rule is useful for assigning some special, internal meaning to percent hack addresses.