Tables & Classes involved in Purchase  Order

PurchTableType and PurchTableLine classes will get called while creating the PurchaseOrders.
PurchFormLetter_PurchOrder
PurchFormLetter_ApproveJournal
PurchFormLetter_Invoice
PurchFormLetter_PackingSlip
PurchFormLetter_ReceiptsList
classes will be used to post the PurchaseOrder at various document status (packing, invoice etc).
Tables:
PurchTable contains all purchase order headers regardless whether they have been posted or not.
PurchParmTable and PurchParmLine contains detailed information regarding posting Purchase headers and Lines.
VendPackingSlipJour and VendPackingSlipTrans tables contains posted packingslip headers and lines.
VendInvoiceJour and VendInvoiceTrans tables contains all invoiced purchase order headers and Lines.
VendReceiptsJour and VendReceiptsTrans tables contains posted receipt header and lines.
VendPurchOrderJour and VendPurchOrderTrans tables contains Purchase requisition headers and lines.