What is a debit note?
A debit note is a document issued to increase the value or the tax of a supply that has already been invoiced — typically when the original invoice understated the price, the quantity or the rate of tax. Under GST it is issued by the supplier, must reference the original invoice, and is reported in the returns for the period in which it is issued.
Who issues it
Under GST, the supplier. In ordinary commercial usage a buyer will often say they are raising a debit note on a supplier for a shortage, but for GST purposes the document that adjusts a supply upward comes from the supplier.
Against the original invoice
A debit note must reference the invoice it adjusts. An unlinked note is a figure the portal and your customer cannot reconcile to anything.
Timing
It is reported in the period in which it is issued rather than the period of the original invoice, so it adjusts the current return rather than requiring the earlier one to be amended.
Versus a credit note
A debit note increases the taxable value or tax; a credit note decreases it. A price increase is a debit note, a return or a discount is a credit note.
Related questions
Is a debit note the same as an invoice?
No. It adjusts an existing invoice upwards rather than recording a new supply, and it must reference the document it adjusts.
Who raises a debit note under GST?
The supplier. Commercial practice where a buyer "raises a debit note" on a supplier does not change who issues the GST document that adjusts the supply.
Which period does it go in?
The period in which it is issued, not the period of the original invoice — so the current return absorbs the adjustment.
More definitions
Link the note to the invoice
tBiz references the original document automatically, so nothing reconciles to nowhere.