|local reference/local platform number|
|Used on these elements|
|Tools for this tag|
So it is used for the reference of one platform, as ref=* would indicate the ref of the whole stop area.
Rendering on Transport Map
Example with three bus stops: https://www.openstreetmap.org/#map=19/51.50814/11.88204&layers=T
Stop Positions of Trams and Trains aren't rendered. The reason is unknown.