XID is a system wide universal ID. It is useful when you want to export data and then import it to another Mango installation.
XIDs add an abstraction layer from the database. In a JSON configuration, references between objects are done using XIDs.
In the actual database, it is done using IDs but from one database to the other the IDs may be different. For example, a Data Points time series is linked to the Data Point ID and not the XID.
It is important to remember that all XIDs must be unique.