The need for this capability was driven to provide simpler handling of multi-source datasets coming from CASA (which have the RA and Dec of the sources given for each visibility).
However, it is NOT intended to be of general use. In practice, it will be reserved for RA and Dec daps, when needed, and UV Tables having such daps are intended to be transformed (split into independent sources, or converted to a multi-field mosaic with only offsets stored) by dedicated applications. For example, multi-source datasets coming from CASA would be imported as FITS files, and converted to GILDAS UV Tables by the GIO library. The resulting GILDAS data sets can then be exploded to single-source UV data very simply.
The capability of having Real(8) also offers more flexibility for the future. However, most daps will still be Real(4) only for the time being.