Groups
A group combines dimension members into a higher-level category. Merge twenty product subcategories into five buckets, or fold misspellings of the same city name together.
A group produces a new dimension whose members are your buckets. It's static in the sense that membership is by name — a new member that appears later lands in Other until you edit the group.
Sets
A set is a binary classification: every member is either In or Out. Sets can be static (you tick members by hand) or dynamic (defined by a condition or a top N rule).
A dynamic set re-evaluates as data changes — Top 10 Customers by Sales stays correct next quarter without you touching it. That's the main practical difference from a group.
Using a set in a view
Drop a set on Colour and you get an In/Out split — a fast way to show "our top customers vs. everyone else". Drop it on Filters to keep only members. Drop it on Rows for an In/Out header.
Right-click and choose Show In/Out to toggle between the binary form and showing the individual members.
Combined sets and set actions
Select two sets, right-click, Create Combined Set. You can then take the union, the intersection, or the members of one that aren't in the other. That's how you answer questions like "who bought in 2023 but not in 2024".
Set actions let a user's selection on a dashboard redefine set membership on the fly, which is the basis of drill-down and comparison interactions that would otherwise need parameters and several sheets.