In Identification types you can define which forms of identification of guests you accept, and select any of those when creating a booking. For example passport, drivers license etc.
Click on "Create new identification type" and type the name of the identification.
This is only used for manual input. Reception will need to open the reservation card of the guest, navigate to Guests Tab and edit the guest information.
In the Identification Type you will have a drop down list with all of the items that you have create.