In mathematics and logic, a set is a collection of items. RFPs [r], for example, are a member of the set of outside counsel management methods [M], so we could write in set notation r ∈ M. Dotted line reporting [d], another management idea, is not a member of the set of outside counsel management methods, so we would write d ∉ M.
If we think about competitive bids [c] and RFPs, they share attributes, so we can write r ∩ c to describe the overlap of those two ideas. If a selection process uses both RFPs and competitive bids, then the union of the two ideas appears as r ∪ c. Venn diagrams depict such intersections and combinations (See my post of Jan. 3, 2007: Venn diagrams; and March 9, 2007: book on history of Venn diagrams.).
Another notation describes sets as being the collection of elements with a specific property. For example, all the management initiatives that require a capital expenditure (i) would be written as set C where C = {i: i requires a capital expense} and the colon is read as “such that.” These notational grammars from set theory let us describe and think about concepts tersely and rigorously.