When an Opening balance is set for a new customer ( new Customer record > Financial tab > Opening Balance), an entry is being created:
Debit: Accounts Receivable XXX
Credit: Uncategorized Income XXX
The system defaults a credit to the Uncategorized Income account as an initial classification for the amount.
The Journal Entry can be edited for the user to reclassify the account from Uncategorized Income to its proper account based on their business process.