Here you can save or freeze selections for the long term and store them in one of three tags. You can save either point, edge or polygon selections. You can then manipulate frozen selections at any time using these tags in the Object Manager.
Point Selection tag.
Edge Selection tag.
Polygon Selection tag.
Existing and selected Selection tags will be overwritten if this command is executed while pressing the Shift key.
You can find out here how to assign separate material properties to frozen polygon selections. Frozen selections are also very important for Deformation objects. More information can be found under Deformation.