Aspire allows you to estimate materials in one unit of measure, and purchase them in another unit. For example, you might estimate roundup for jobs in ounces, but order from the vendor in one or two-gallon containers. In this case, ounces represent the allocation units, and one gallon or two gallons represent possible options for purchase units.
The article Purchase Units vs. Allocation Units explains how unit types are referenced and used in different parts of Aspire.