When do I use a Debit Note ?

You under-billed the customer. You sold them goods worth $110/- but mistakenly issued the Invoice for $100/-. You could then issue a Debit Note to them for $10/- to sort this out. Note you could also issue another Invoice for $10/- if you prefer. You should use the same Account you used in the original Invoice. You could enter text the body of the current Debit Note to let them know which Invoice you are referring to.