There is now a generalized keyword mechanism that can be used to group related items. It's available in the theme field. It uses hierarchical keywords; they look like pathnames.
The generalized keyword mechanism was used to experiment with choosing the categories described earlier. Even though the three categories have become separate fields in the work records, this mechanism will remain, as it can be handy for ad-hoc situations. E.g. it was recently used to associate multiple levels of budget spending with the associated work items, as an aid in determining what to spend next.