Open SMD File
Below are two possible types of uses applicable to SMD file extension:
Valve StudioMDL Data File
The SMD file is a three-dimensional model used by the Source Engine, a game engine used to develop games like Half-Life.
The SMD File stores the model in an ASCII text format, and compiles it into the resulting MDL game model.
Sega Mega Drive ROM
In this application, similar to the previous one, the SMD file is a Rom of a Sega Genesis video game and can be played on a PC using a Sega Genesis system emulator.