Altova website: UML Interaction Overview diagrams
Interaction Overview Diagrams are a variant of Activity diagrams and give an overview of the interaction between other interaction diagrams such as Sequence, Activity, Communication, or Timing diagrams. The method of constructing a diagram is similar to that of Activity diagram and uses the same modeling elements: start/end points, forks, joins etc.
Two types of interaction elements are used instead of activity elements: Interaction elements and Interaction use elements.
Interaction elements are displayed as iconized versions of a Sequence, Communication, Timing, or Interaction Overview diagram, enclosed in a frame with the "SD" keyword displayed in the top-left frame title space.
Interaction occurrence elements are references to existing Interaction diagrams with "Ref" enclosed in the frame's title space, and the occurrence's name in the frame.