Hi Siavash,
I’m figuring that you’re using the .NET binding provided with the Unity SDK. Yes, the documentation is a bit scarce at the moment; we’re working on it for later releases.
But please do have a look at the Developer’s Guide included with all EyeX SDK packages (and available on the documentation page. It should provide answers for at least some of your questions, at least the big ones such as what a snapshot is and what the capabilities of the EyeX API are.