UD Mappings are associations between columns displayed in the file list and UDTIs or main window named filters. The supplied name of a mapping 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 Mapping # (name)
for a UDTI or
UD Mapping # (⛉ name)
for a named filter. # is 1, 2 or 3. name is the name of the UDTI associated with the column.
UD Column Mappings associated with UDTIs are an older construct which predates the implementation of Custom Fields. Custom Fields are much easier to access from with actions.
When a named filter is associated with a main window column, the filter is called to determine a value. A result of false displays as empty. A result of true displays as ✓. For example if you create a filter named has Lyrics defined as:
Any - Lyrics - not equal to - empty
and you associate the filter with a UD Column Mapping, the main window will display ✓ for all tracks which have lyrics, in the column.