Customization in Purchase Invoice

What are the changes required in Purchase Invoice if it is linked with some other doctype other than ‘Item’ doctype?

@erpnext_user what kind of changes are you referring to?

We have another kind of doctype apart from Item doctype for which we want to create an invoice which includes purchase and sales invoice).Now purchase invoice is linked with supplier and items.So, we meant to say that how should we proceed in linking the purchase and sales invoice so that the PL sheet and Balance sheet calculation is not compromised. Any help from your side would be appreciated.