The extraction functionality allows extracting a selected part of a sequence diagram to a newly created diagram. After extraction, an interaction use is created in the source diagram in place of the extracted part(s).

That is, the interaction use is a referencing element that references the newly created sequence diagram.

As you can see in the example, during the extraction, the selected part of the sequence diagram is moved to the newly created UserSession sequence diagram. The interaction use element that references the UserSession interaction is created in the source diagram.