Shader keywords determine which shader variants Unity uses. You can use a GlobalKeyword to enable, disable, or check the state of a global shader keyword. For information on working with local shader keywords and global shader keywords and how they interact, see Using shader keywords with C# scripts.
To set the state of a global shader keyword, use Shader.SetKeyword, Shader.EnableKeyword, or Shader.DisableKeyword. To get all the global shader keywords that exist, use Shader.globalKeywords. To get all the global shader keywords that are enabled, use Shader.enabledGlobalKeywords.
In addition to this, you can also enable or disable a local or global keyword with a CommandBuffer. To do this, use CommandBuffer.SetKeyword, CommandBuffer.EnableKeyword, or CommandBuffer.DisableKeyword.
Additional resources: Shader variants and keywords, LocalKeyword.