Answer:
It is likely a computed field.
Step-by-step explanation:
A computed field is a field in a database table that is automatically generated based on the values in other fields in the same record.
In this case, the CheckOutID field may be automatically generated based on other fields in the same record, such as the date and time of checkout or the ID of the item being checked out. The field may be computed using an expression or formula that takes the values in the other fields as input and generates a unique ID for the checkout.