page sets
Multiple page sets can be used when a company has multiple distinct websites but more usefully a single page set can be shared by several domains which will present the same page data under different domain names. As a distinct domains can have distinct themes this can be used to style the same pages differently either for stylistic reasons or to provide alternative layout for desktop or mobile devices.
A page does not need to be connected to a domain so can form a simplistic backup, snapshot or to make multiple changes in one atomic update, eg, clone the active pages, make multiple edits, test on a development domain, when ready attach to the production domain to publish. Unattached page sets are not viewable so it is useful to have spare domains names, eg, in addition to www.example.com use test.example.com, newsite.examples.com, etc.