RE: Amount in the sales header
It is used to calculate the invoice discount of the sales order. To preserve database performance during sales order entry, the Amount field is only updated when a calculation of the sales invoice discount is required, in the following situations:
When you post (invoice) the sales order partially or fully.
When you calculate the sales invoice discount manually.
When you open the Sales Statistics window for the sales order.
When you release the sales order.
In fact, this field doesn't really exist in the table. If you want to calculate the Sales amount for reporting purposes, Sum the [Amount] or [Amount Including VAT] field from the Sales Order Line table.