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"Use Edit Log for transactions/masters, and digital signature for invoices/reports with Release 2.1. Download NOW!"
https://help.tallysolutions.com/docs/td9rel54/faq/6208.html

Validating the content of a Form in Edit/Create Mode

The Form attribute Control can be used to validate the contents of the Form.

For example :

[!Form: Normal INV Voucher]

Control: BilledQtyNegative: $$IsNegative:$$CollQtyTotal:AllInventoryEntries:$BilledQty