Aggregation Level & Selection ID in Demand Planing Background job

Folks

Whle creating a Demand Planning job in the background, we need to make few selection such as

1. Choosing a Selection ID(One, Several & Select All) and

2. Choosing an Aggregation level.

My question is : Doesn't the selection ID already have the aggregation level defined with in as selection ID is basically something defined or shown as Product by Location and then by Sales Org etc. Isn't this the same as the aggregation level ???

Why do we need to have Selection Id as well as Aggregation level in Background job ?

In the interactive planning do we need the aggregation Level ? If so where do we specify the same ?

TIA