Allocation 14.1 Need at Hierarchy level

In creating an allocation - typically a what-if, but the question applies to standard allocations as well - if the user selects subclass, class, or department, or makes a user-defined selection (such as an item list) and chooses history, how does the allocation calculate need based on a given SKU?  In a what-if, if the user does not enter a specific available quantity, the tool assumes infinite quantity.  How does the tool review the need at the hierarchy level and bring it down to just this one SKU/store combination?  We are using Allocation 14.1.