Uses a player build optimized to run as a headless server.
When StandaloneBuildSubtarget.Server is selected, Unity compiles the standalone player for each target platform as follows:
MacOS Player: Compiled as a standard console application.
Windows Player: Compiled with /System:Console and runs as a standard Windows console application.
Linux Player: Compiled as a standard console application.
Unity defines UNITY_SERVER when it compiles scripts for a dedicated server build.