We use ledger accounts to categorise and group your purchases and other transactions. For example, if the invoice is for stationery, you might use a ledger account for
Office Supplies.
When you save the invoice, the net value (without VAT) records against this ledger account. This is so you can report on the total amount spent on office supply sales.

When you create an invoice, the default set on the supplier record appears first. This is to help you make sure you use the right ledger account on your purchases automatically.
If you add a product or service to the invoice, the ledger account gets taken from the one saved on record.
You can change the ledger account shown on any of your invoice lines.
Read more about ledger accounts