Atari Subs is an early Atari Raster game. It had a color overlay, which is emulated by a PNG file in the overlays directory.
This version was ported to MiST by Gehstock and MiSTer by alanswx and JasonA
The original game had two displays, and a head to head combat mode. Given the limitations of MiSTer this just shows the right screen and offers a 1 player game with computer opponent. The VHDL simulation supports two players, but it isn't presented to the user.