Do you mean Categories or Headings/Subheadings, Barry?
If you mean Headings/Sub-headings, you can use any reasonably descriptive name you choose for your optional subheadings.
I sometimes divide my See also: citation list by type, using subheadings for Birth, Marriage, Children, Death, etc. In fact, I usually start with FamilySearch.org as my first subheading; I then use the other subheadings under the FamilySearch.org subheadings. If I link to Findagrave.com, BillionGraves.com, Ancestry.com, genealogy publications, periodicals, books, etc., I use subheadings for the citations from those sites.
If you mean Categories, they, too, can (and should) have any reasonably descriptive name that fits their use. However, their use and names, other than for personal categories, should meet the overall needs of the community.
How you use subcategories is, in my opinion, more up to your personal preferences, within reason.
Here is a recent example of my subheading work: