For the first register assigned a particular code, the balance defaults from the entry journal for this account code—petty cash or bank—or you can enter an amount.
For subsequent registers assigned a particular code, the open balance defaults from the Closing balance of the previous register and you cannot modify this amount.
For example, if Cash register CR#1 is assigned Petty cash account PCABC, the opening balance is taken from the journal entry for this account. If you close CR#1 and assign CR#2 to the same PCABC code, the opening balance for CR#2 is the closing balance for CR#1.