Regular shadow maps compute a depth map from the point of light. The shadow shading language can be used to check the depth of the point in the same view against the depth stored in the map to figure out if the point is in the shadow or not. Unfortunately, regular shadow maps store only depth. So it is not possible include semi-transparent objects in the shadow. Transparency Shadow Maps (TSM) fix this issue by storing a transparency as a function of depth for each pixel. They allow many semi-transparent objects to cast correct shadows.
To compute a TSM you need to use the "tsm" display driver:
Display "test.tsm" "tsm" "z" - Compute TSM
Display "test.tif" "file" "z" - Compute regular shadow map
The extension of the TSM file must be .tsm. Otherwise, the renderer can get confused and may try to read it as a regular shadow map.