From memory (it's more than 10 years since I worked on an aircraft with a Stormscope) but it is, in effect, a 10(?) Mhz ADF that detects the fields from lightning strikes. The original Ryan/3M Stormscope gave finer detail than the Honeywell version but the larger symbols of Brand H made assimilating the data/avoiding the strikes easier.
The range display effectively also changes the wash-out filter on the display such that you see only the latest strikes or an appopriate number for the scale.
Again this is from memory but may be of help.