This is the Compustar FT-DM800 positive to negative input adapter.

Compustar systems read negative inputs, for the most part. Where a vehicle provides a positive signal instead, for example a positive ARM input, this adapter converts it into something the Compustar can actually use.

It saves building the same conversion out of a relay and a harness, which takes longer and adds another point of failure under the dash.

Watch for a detailed explanation:

Converting an output rather than an input? See the FT-DM600 and FT-DM700.