To change the behaviour for Drop Shipments, open the "Purchases & Payables" page and click the dropdown for the Remove Link to Sales Order field.
Here you have 3 options:
'Default' - Makes no change to the drop shipment functionality so any lines that are ticked as Drop Shipment on a "Sales Order" and "Purchase Order" remain connected when receipting/shipping so both documents are updated at the same time. The "Sales Order" has to be Invoiced prior to the "Purchase Order" will in turn post both documents together.
'Automatic' - The receipting/shipping process remains the same, however once that is done, the link between the 2 is removed meaning the "Purchase Order" can be invoiced first with no prompt or reminder to the User. This will leave the "Sales Order" to be invoiced manually.
'Automatic with Prompt' - The receipting/shipping process remains the same as standard, however once that is done, a confirmation message will appear advising the User to confirm they want to remove the link between the 2 documents when trying to post the "Purchase Order" first. If 'No' is selected, than it reverts back to standard functionality (the sales and Purchase documents remain connected and the Sales Order must be processed first). If 'Yes' is selected, then the link is broken between the 2 which means the Purchase Order can be invoiced first and the Sales Order will remain open requiring manual invoicing.