I have two applications, A and B, both with a web-based user interface.
Both applications are integrated via an URL iframe mechanism.
A user can click on a link in application A and then gets the UI of application B as am iframe in application A.
Now, since both applications have an interface between each other (do they?): Who provides the interface and who requires the interface, in the UML sense? What is the main information flow on this interface?
IMO it depends on the detail level of UML. In a use case diagramm (abstract top level) you would usually show what an actor does with a system.
The "interface" (GUI) used by the actor is composed of your systems A and B, possibly the integrating application C adds some additional functionallity.
In an activity diagramm you could model how data is passed around the systems.
Back to your question: In a component diagram you could show that A,B provide an interface which is used by C (composed).