Purchase Request

You can turn on the Purchase Proposal add-on (Tools -> System -> Add-ons), and this will allow users to create purchase order proposals (requisition requests) which don't have the official status of a regular PO and won't be included in On PO quantities for items. 

You can require that certain users can only create purchase proposals by turning on the user setting Admin -> System -> Users -> User Rights For Company -> Can Only Create Purchase Proposals Not POs.

You can convert existing POs to Purchase Proposals by editing the PO and clicking the Convert To Purchase Proposal button at the top of the page.

You can convert Purchase Proposals to Purchase Orders one of two ways. You can click the Convert To Purchase Order button at the top of the page when editing Purchase Proposals. You can also go to Expenses -> Purchase Orders -> PO Proposal Management and from there convert proposals to orders.