A purchase requisition and a purchase order are frequently used interchangeably and are two different acts performed by different people at different moments. The requisition asks internally for permission. The order commits externally to a supplier. Which comes first is not a formality: it is the whole reason the approval means anything.
The requisition
Raised by whoever needs the thing, stating what, roughly how much, which budget, when and why. Approved by a budget holder deciding whether the organisation should spend the money. It commits nobody outside the organisation and can be declined at no cost, which is precisely what makes the decision real.
The order
Raised by whoever handles buying, having chosen a supplier and agreed a price, and sent to that supplier. It carries a unique number, the lines, the delivery expectation and your terms. Once the supplier acts on it a commitment exists, which is why approval belongs before it goes out rather than when the invoice arrives.
Why organisations conflate them
In small teams the same person often does both in one action, which is fine as long as the approval still happens before the supplier is told. The conflation becomes a problem at scale, when the person raising the order is not the budget holder and nobody notices that the approval has quietly moved to after the commitment.
Questions people ask about purchase requisition vs purchase order
Do we need both documents?
You need both decisions. In a small organisation they can be one action, provided the approval genuinely precedes the order going out.
Who approves which?
The budget holder approves the requisition. Purchasing issues the order, within the approved value and their own authority.
What if an order is raised without a requisition?
Then the spend decision and the commitment happened together, which is workable at low value and weak at high value.