"Xero sales account" is where the invoices are uploaded, they may not have been paid yet but they have been uploaded to this section. An example name might be: 200 - Sales.
"Xero Payment account" is where your money actually goes once the invoice in this sales account has been paid. An example name might be: 160 - Revenue.
"Invoice due in" is where you select how long you would like to give your patients to pay their invoice.