Owners ask for unit prices where quantities are uncertain. The bid form lists the items. The final payment is the unit price times the measured quantity.
Site work uses them often. Rock removal, unsuitable soil replacement and extra pavement patching are common examples.
A unit price must carry everything: labor, material, equipment, overhead and profit. Small quantities cost more per unit than large ones.
Read how the documents measure each unit. Check whether the same price applies to adds and deducts.