A substitution of the form ${mapping,argument} is handled specially. The mapping,argument part is used to find and apply a mapping from the IMTA mapping file. The mapping field specifies the name of the mapping table to use while argument specifies the string to pass to the mapping. The mapping must exist and must set the $Y flag in its output if it is successful; if it doesn't exist or doesn't set $Y the rewrite will fail. If successful the result of the mapping is merged into the template at the current location and re-expanded.
This mechanism allows the IMTA's rewriting process to be extended in various complex ways. For example, the username part of an address can be selectively analyzed and modified, which normally isn't a feature that the IMTA's rewriting process is capable of.