Subflows are fully supported by the debugger. The iSM rule for a subflow is that it must end by returning a single document. The name of the End node of the subflow becomes the edge name followed by the calling service.
Subflows are deployed as system flows, independent of a specific channel.
You can step into a subflow if you are on the calling service. You can also set breakpoints and watchpoints in the subflow by first setting the source to the subflow.
If you are currently executing in a subflow and desire to simply complete the flow, a simple approach is to: