Modifying sales order

Hello,

Maybe it will looks annoying, but can someone help me share or explain about code in sales order entry? I must heavily modify the code and I just can not understand what is the code meaning. It's very compilcated because one file contain many function, from create sales quotation until modify sales and anything else. I just can not understand where is the function to display the form (because I must add some input text) and where is the function to save the data into database (the query insert or update).

Basically, my modification is to crate some contract based sales order and some identification whether it is from PO blanket or ordinary PO. I think I must add 2 dropdown, one from the contract and one from the identification. The modification is simple but because the code is very complicated, I can not do anything this 4 days.

Plese someone help me or at least give me some clue.

Thank you.