UD Text Mappings are associations between columns displayed in the file list and UDTIs. You specify the name of the UDTI to associate with a mapping. The supplied name is also used as the column header.
When looking at the context menu associated with the main window's table, these columns are displayed as: UD Text Mapping #(name). # is 1, 2 or 3. name is the name of the UDTI associated with the column.
UD Text Column Mappings are an older construct which predates the implementation of Custom Fields. Custom Fields are much easier to access from with actions.