The documentation for @output_source is quite poor

[pmantica1]

The documentation for @output_source is very poor. First, it is unclear why it's named @ouput_source. Second, Its section the README.rst points to Completeness of returned results which gives a hand-wavy explanation of what it does. Finally, one of the constraints is quite unclear :
"can exist only on a vertex field, and only on the last vertex used in the query." It is unclear here what "last means".