what is stock requisition?

It's the process by which, in a large organisation, those who need specific items (pens, chairs, envelopes, etc.) obtain them from the central department that is responsible for purchases on behalf of that organisation.

For example - I need a box of pens for myself and my office colleagues to write our notes. Rather than me going off to the shop and buying some at the full retail price, I requisition some from the stock (bought with a bulk discount) by my firm's Purchasing Department.

The phrase does, however, have other meanings that are particular to a specific work environment.

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