If there's only one CU and it's not already labelled, print a label marked "DB1" and stick it on (or write directly, not so tidy). Then write "DB1" in the designation box.
The reason it's there is so that there's no doubt about which box you're referring to. If multiple boxes on the premises, carry on labelling "DB2", "DB3", etc, or whatever your preferred labelling scheme is.
On larger systems, I have used, for example, "DB1.3" for a DB supplied from CB3 on DB1, but maybe that's over-complicating things! (Does make it easy to see where something is supplied from though.)