Actual cost only tells you what has been invoiced or paid. A commitment is cost you have already agreed to: a signed subcontract or a purchase order. Commitments tell you what is coming, so they belong in every forecast.
For each cost code, the useful split is: cost to date (invoiced and posted), open commitments (committed but not yet invoiced) and uncommitted budget (scope you have not bought out yet). A forecast that puts cost-to-complete below the open commitments on a code is almost certainly wrong: you already owe at least that much.
Buyout is the process of turning estimate lines into commitments. Buying out below budget creates buyout savings, which are real only when the commitment is signed and the scope is complete in it. Commitments change too: a change order with the owner usually means a matching change order to the sub.
On the payables side, the three-way match checks each invoice against the commitment (PO or subcontract) and against what was received or completed. An invoice that exceeds the remaining commitment is a signal to stop and ask why before you pay.