Definition of Interface Agreement
An interface agreement is a document that defines an interface between two teams / locations / functional responsibilities.
It may be signed by both parties when agreement is reached, but not always.
Used more by stakeholders that “own” part of a project, rather than between contractors (who refer to their contract) - but see the green note below.
An Interface Plan is an agreement between interfacing parties regarding scheduled access to 'contract objects' prior to load out and delivery - for example, for testing of subsea control units, umbilical, etc.
Note that the Project Interface Management Plan is a core project document that defines how interfaces will be managed, and is described further here.