Whether It is PO or SO first the FormLetter class will be called and then based on whehter it is a PO or SO
the PurchFormLetter and SalesFormLetter Classes will be called. and then based on the stage of posting it will call different classes.
Eg: If you are positng a PO invoice PurchFormLetter_invoice class will be called.
If you wan to know the what are all the different classes and methods that are called during SO/PO posting - put a break point in FormLetter - new method
Infolog is used for displaying the information whether it is an error or info or warning.
where as dialog helps in giving some inputs for the process to happen. eg: Yes No selection and dialog during the reports.
cust,vendinvoicejour has some initfromsalestable method.the form letter class call this method and it will insert the header level records(sales table) to cust invoice jour and footer level(saless line) records to cust invoice trans(here it will call initfromsalesline).