The Rate-based cost as well as the Per-use cost operate in different ways, depending upon whether the resource is a work resource (a person, rental equipment etc.) or a material resource.
The Standard Rate cost is Applied per hour for Work Resources and per unit assignment for Material Cost.
The Per Use cost, for Material Resources applied once per assignment and for Work Resources depends on the assignment units used.
A resource can have both a rate-based cost and a per-use cost set up