When building your review request email trigger you can opt for either Placed Order or Fulfilled Order. However, Fulfilled Order as a trigger provides better insight regarding when received the product thanks to the shipment tracking.
It is important to note that the timing of the sent review request email should leave enough time for customers to use and evaluate the products that they purchase. That’s why the default ‘product review/cross-sell’ flow uses the Fulfilled Order as a flow trigger.
On the other hand, you can use filters to create different flows based on purchase value, the number of items purchased, previous purchased, etc.