Goods and/or services must not be provided on credit without a customer purchase order (or the appropriate written equivalent contract). Many organisations will not process payments without such documentation and may cause settlement to be unnecessarily delayed.
A customer purchase order provides written confirmation of the customer's details (e.g. current address, contact details) and can be referenced on the invoice raised.
This is particularly important when arrangements may be informal, such as sponsorship. Formal written orders avoid problems at a later stage. Note however, that a purchase order will often include reference to the purchaser's standard terms and conditions, so that before matters are concluded an acknowledgement should be sent which makes it clear that the University's STCs will apply (see Ensuring that University STCs apply).
For sales where the University's Standard Terms & Conditions are to be applied the customer's purchase order must be accepted in writing (inc. email).