Contracting Guidelines - When is A PO alone sufficient, when is contract required?

Hello;
Procurement is currently working with our Legal team to develop a 'Contracting Playbook'; one of the areas to be covered is addressing the question we often hear, especially within the Services commodity areas, 'When do I Need A Contract'? Procurement Policy dictates that a PO is required for any goods and/or service purchase, but we have never been able to define when solely a PO will suffice and when a contract is additionally required - we understand it is not something that can be a 'hard and fast' rule, and cannot be determined by category or spend level alone; risk is a large factor to be considered. We are trying to put together general guidance.
Has anyone developed any type of guidelines addressing contracting for the Indirects spend areas?

I greatly appreciate any assistance/recommendations. Thanks very much in advance for your input!

Michelle