What is user ID actually used for in Optimizely isFeatureEnabled()?

I'm using the C# SDK v4 and Feature Experimentation.

I'm struggling to understand what the userID is actually used for in IsFeatureEnabled().

Can we create a rule that says if user ID = 123 the feature toggle is enabled, otherwise it's disabled?

Do we need to some how import all our user IDs into Optimizely?

Is the userID case sensitive?