A requisition is a request for something, especially a formal written request on a pre-printed form.
An online requisition is an electronic document, which can be originated by the requester and then using the company workflow or hierarchy rules, can be submitted to the subsequent levels, until it is finalized or approved and then converted to a purchase order. While reviewing the electronic requisition, the reviewer should have the ability to check different parameters of the requisition with regard to the inventory levels and financial controls.
Requisitioning is sometimes a compulsory act similar to expropriation; for example in wartime some governments may (compulsorily) requisition what they need.