The terminology around procure to pay is a minor irritation that causes real confusion in evaluations, because vendors use the terms loosely and buyers assume a shared meaning that does not exist. Four terms, briefly settled, so that a conversation about scope is about scope rather than about vocabulary.
Procure to pay and purchase to pay
The same thing: the cycle from a requisition through the purchase order, delivery and invoice to payment. P2P is the abbreviation for either. Different vendors and textbooks prefer different phrasings and no distinction should be inferred from which one somebody uses.
Source to pay
Wider. It adds the sourcing stage at the front: identifying suppliers, running tenders, negotiating and contracting, before any requisition exists. This matters for organisations with a purchasing function and is usually irrelevant to a finance team whose suppliers are already chosen and whose problem is downstream.
Order to cash, and what it is not
The mirror image on the selling side: from a customer order through delivery and invoicing to cash collected. It is not part of procure to pay; it is what your supplier is doing while you are doing procure to pay. Understanding it explains a good deal about how suppliers behave and what they need from you.
Questions people ask about procure to pay p2p
Which term should we use internally?
Whichever your team already uses. What matters is agreeing the scope in a sentence rather than assuming the label carries it.
Do we need source to pay?
Only if supplier selection and contracting are a real workload. If your suppliers are settled, the sourcing modules are configuration without benefit.
Is P2P always one system?
No. Plenty of organisations run point solutions with an integration, which is a reasonable architecture with a different set of trade-offs.