'Spot Buy' feature in 12.2.7 vs Emergency PO feature from earlier versions

Can someone explain the difference between the new 'Spot Buy' feature in 12.2.7 for iProcurement vis-a-vis the Emergency PO Number feature using the 'I need a Purchase Order number immediately' option?

Both of them appear to use the Buyer assignment for the category; both of them require the supplier to be assigned and both of them require the Requisition to be approved. However, Spot Buy has a grant security requirement that seems to add additional layer of complexity. I can see that the Spot Buy feature allows for an upper limit on the total amount by Category. Except for this amount limit, both features appear to be allowing the PO to be created automatically.