missing tag `extra` in include of elements

I've noticed that some tests use an additional element inside the include element called ``, which is not documented on the website and which is missing from the respective `.sdf`'s. These seem to occur in test sdfs of the format `X_include_with_interface_api.sdf`. Is this an undocumented feature of SDF, something internal that should be ignored, or an old test?

Example: https://github.com/ignitionrobotics/sdformat/blob/sdf11/test/sdf/world_include_with_interface_api.sdf