what is the rv!identifier

1. the rv! domain is 'Record Variables' so only applies when referencing Appian Records
2. a Record will always have a value that uniquely identifies it (e.g. if the Record is Entity-backed - that is, uses a database table as its source) then the Primary Key for that Entity is the unique value for that Record
3. the Primary Key in the CDT that maps to that Entity can be named something like 'CustomerId'. 
4. rv!identifier is effectively a synonym for this unique value.

For example: if you wanted to start a Related Action for a given Record Type you could pass into that Related Action the Primary Key value of that Record, and use that to retrieve the latest data for that record into that process model. You have a design choice as to whether to reference the Primary Key by its CDT name - CustomerId in the example above - or by the fact that it has a synonym called rv!identifier. I would strongly recommend the latter as there are no guarantees that the name of the Primary Key couldn't change, which would mean your reference to it would break; whereas the rv!identifier would ALWAYS be true.