An edge may not leave a hidden output port

What it means

R7.b's "not exposed" is satisfied without any instance ever touching the port: the error output every executable node carries ships hidden by default, so an edge leaving it is refused on the type's own default alone, with no override in play at all. An instance override is one way a port ends up not exposed; a type default nobody has changed is another, and R7.b treats them alike (R7.c decides which applies).

Example

The source node's type declares its error output; this instance never overrides the default that hides it:

{ "id": "e1", "source": "s", "sourceHandle": "s-output-error", "target": "t", "targetHandle": "t-input-value" }

{ "code": "R7_EDGE_SOURCE_NOT_EXPOSED", "locator": "edge.0" }