Mark as seam

Selected edges or the outlines of a polygon selection can be defined as Seams and saved to the object as a Seam Selection Tag. Seams are evaluated by certain UV functions and then interpreted as cuts or openings in the UV unwrap. This allows you to define specific UV islands and place cuts through the geometry to calculate a flat UV unwrap.
The same command is also available directly from a Seam Selection Tag.

Also have a look at the Seam Selection tool for additional seam selection options.