SalesTableType and SaleslineType classes will get called while creating the orders.
SalesFormLetter* classes will be used to post the sales order at various document status(packing, invoice etc).
SalesParm* tables are used to prepare the data for posting
CustConfirmJour, CustConfirmTrans - when a sales order gets confirmed
CustPackingSlipJour, CustPackingSlipTrans - when a packing slip is posted.
CustInvoiceTable,CustInvoiceTrans - when an invoice is posted.
These are some of the maily used tables.