This is a subsidiary template to {{BS-overlap}}. This template takes a single argument, which is the name of a route-map component, and produces alt text for that component. Please see Wikipedia:Alternative text for images for more about alt text.
To describe the icon "File:BSicon ID.svg", add a line:
|ID=Description of the icon
to the template. The ID is a pictogram ID; see Pictograms. An example pictogram ID is utSTRlg, which identifies the image File:BSicon utSTR+r.svg 
; this ID has the prefix "ut", the root "STR", and the suffix "lg". The corresponding entry in this template's source code is:
|utSTRlg=Urban tunnel from right
Directions in alt text are written from the driver's viewpoint. The driver faces down the page; in this example the tunnel comes from the driver's right and then goes straight ahead.
Entries in this template should be sorted first by core (using a standard en_US locale), then by prefix, then by suffix.