The Identifier column on the Doorway table and in the Downloadable CSV is how admins identify the different Doorways that a single User may own so that they can separately manage each Doorway (the information it shares and its assigned Template).
The Identifiers for each single user's Doorways follow the same pattern:
Doorway 1 = "Primary"
Doorway 2 = "Secondary"
Doorway 3 = Doorway 3
Doorway 4 = Doorway 4
Doorway 5 = Doorway 5
etc.
Combined with the Doorway Name, this feature allows admins to manage multiple Doorways for each user at scale.