The 'INTERFACE' operator return value is used to indicate where an operator has successfully completed by opening a menu or other UI component.
The comment in the source code for the operator return types states that it is:
/* "used for operators that act indirectly (eg. popup menu)" */
/* "note: this isn't great design (using operators to trigger UI) avoid where possible." */
This new return type was added during Commit - Fix T41041: "'Delete keyframe' removes markers too"
Reason for commit update was:
"Operators that trigger UI events (but nothing else)
were using 'CANCELLED' making it impossible to tell if an invoke
function failed, or opened a menu."