Spend Requests is for asking to spend money before it is spent. A request is totalled from its lines with VAT, checked against the budget for its cost centre and category, and sent to whoever your approval limits name for that cost centre and amount. Larger amounts can need a second approver. When a request is approved its money is committed on the budget line, so the same money cannot be approved twice.
A request can carry supporting documents, such as the supplier’s quote. Approvers see them when they decide, and they stay on the request as evidence: documents can be added later but never removed or replaced.
Anyone in the workspace can raise a request. Approvers are not a fixed role: they are the people, or the holders of a role, named in your approval limits.