The repeatmappings system in the data exporter allows one to reuse existing export mappings in multiple locations in an export. This is useful for export formats where the same data structures are used in multiple locations. (For instance, in EAD where the same record structure is used for arbitrarily deep hierarchies.)
The current implementation only allows reuse of mappings that are actually in-use parts of the export format, and only after they have been defined. That is, anything you need to repeat needs to be defined earlier in the format than where it's referenced and will be used as part of the export format in that first location.
A new mapping "template" row type should be defined (in addition to the current "mapping", "constant" and "variable") that allows definition of rows that are not part of the export format, but are available for use with repeatmapping. This would allow definition of repeatmapping "templates" in clearly defined blocks of rows at the top of a format definition that can then be referenced later on in the format, and should improve readability and maintainability.