What are the classes involved in Journal, Sales order Invoice, Purchase order invoice posting?

Journal Posting: LedgerJournalCheckPost

Sales Order Posting: SalesFormLetter, SalesFormLetter_Invoice

Purchase Order Posting: PurchFormLetter, PurchFormLetter_PurchOrder, PurchFormLetter_ReceiptList, PurchFormLetter_PackingSlip, PurchFormLetter_invoice