(As on date of beginning of books)
If yours is an existing company whose books you are putting onto Tally, this would be applicable in the following circumstances:
If the ledger is an asset or a liability and if it has a balance in the account as on the date of beginning of books in Tally.
Tally recognises normal accounting principles of debit balances for Assets and credit balances for Liabilities. Of course, it accepts the reverse for obverse balances. Revenue accounts normally do not have balances. Tally, however, permits you to give balances even for such accounts - You may be transferring your books on to Tally in the middle of the year and may not have closed them in your earlier system. Hence, you may specify whether the balance is Debit or Credit. Simply D or C would suffice.
Tip to speed up data entry: You may use Ctrl key +A at the field following which the information in other fields do not need to be changed.