If it helps the filter operator and filter run methods only came to tiddlywiki recently, the subfilter operator was around a lot longer.
In a recent reply I demonstrated the subfilter use, see here Keeping track of fields. What are best practises? - #14 by TW_Tones in this case following a list of titles rule out -[subfilter<subfiltername>] or rule in +[subfilter<subfiltername>]
As a result, I now tend to use them according to both their features but also if they sound right in English. Eg subfilter is more of a subset, where filter is a set.