The most common use for this feature is setting the rabies tag number for a patient while you enter the rabies vaccination record. To set this up, start by going to "Inventory Settings" --> "Inventory Types" and open (for example) the Rabies type:
Selecting "Rabies Tag ID" as the "Associated Patient Value" will connect this type to that attribute.
Now when you add any rabies vaccine to an encounter, you'll see this additional entry field:
The "Rabies Tag ID" will be pre-populated with any current value for this patient, and if you update the value to show the latest tag number, that will be stored in the patient record and included whenever you print or save a vaccine certificate.