In v345 we introduced a feature to allow arbitrary fields within bigBeds to be used as labels in hgTracks.
For example, placing "labelFields fieldName1, fieldName2" in trackDb.txt in a hub allows the option to display those fields on the Browser via choices on that track's Track Settings page.
This can be useful if a track has specific information in the additional fields like transcript IDs or gene IDs or other meaningful accessions, that would be desired to display on hgTracks beyond what exists in the name field. If using bigGenePred, such an example that might be useful is "labelFields name, name2, geneName" where name2 and geneName might have these additional pieces of information.