Accessing properties in custom action

[mug-cat]

Rookie question, new to the world of windows installers. Couldn't find any meaningful documentation / information on the internet.

How may I obtain the 'SourceDir' property that is set by the ResolveSource action in the C# custom action code? I've made ResolveSource execute before my custom action and I can see from the installation log that the properties are added.
Neither does setting execution to immediate and session["SourceDir"] nor deferred and session.CustomActionData["SourceDir"] worked for me. They both logged empty string. Is there any official documentation on how I should use the session object?

Lastly, is there any significance and behavioral difference between installer-set properties, wix-set properties, ones I set with Property elements and properties I set in custom action (if I can do so)?