$Project varialbes?

I'm following a course in SSIS from PluralSight. In one module, an Execute Package task references a variable in the parent task like this:@[$Project::foo]where "foo" is a Project variable (new with SQL Server 2012)I'd like to know: 1. Is this usage documented somewhere?2. are there other variable groups that can/must be accessed with the "$" format? e.g. can I refer to varialbes with names like @[$Package::Name] or @[$Parent::Name]?I'm looking for some reference docs on this.