Most of the sources in question are Book Sections (i.e., chapters in an edited volume, where the edited volume appears in a subseries of a series – sometimes numbered in both the main series and the subseries, sometimes only numbered in one or the other).
I’m generally a bit leery of creating new field types (I assume you mean using the numbered Custom fields?), since those are ‘shared’ by everyone, as it were: in a library as big as the one I’m working with – about 6,000 entries, most imported from other EndNote libraries – there’s a good change someone has used any given Custom field for some other purpose, so I risk adding some random piece of irrelevant information to my bibliographic entries.