Power domains are the fundamental parts of UPF constructions. All other predominant factors that are used to completely define the UPF are established around the power domain boundaries. Hence, the UPF power domain is a collection of instances that are treated as a group for power-management purposes.
The interface of a power domain is the union of the upper boundary and the lower boundary of the power domain. This article from Mentor Graphics describes the importance of UPF power domain and domain boundary.
Find out how T&VS provides a methodology for incremental specification of power intent using UPF.