A field whose values come from a list you write, for departments, statuses, product categories or anything a standard field type does not cover.
Standard field types cover names, emails and dates. They do not know your departments, order statuses or product categories. Custom fields do: write the list of values, and the generator picks from it for every row.
Value lists are still part of every schema in RNDGen, and AI data can now write them for you from a description.