You can assign Orders permissions in Access Management.
Yes. Open the order in Edit mode → go to Order settings (Cmd/Ctrl + B) → Document Defaults → Details → Purchase Order
Configure the reference prefix and series based on your preference

Open the order in Edit mode → go to Order settings (Cmd/Ctrl + B) → Document Defaults → Details → Order
Enable Request → Order control to prevent orders from exceeding the request value
Example: If a Request is PHP 500, converting it to an Order of PHP 800 will be blocked when this control is enabled.
Enable Enable Order → Bill control to prevent bills from exceeding the order value
Example: If an Order is PHP 500, converting it to a Bill of PHP 800 will be blocked when this control is enabled.
