At Level 0 (zero), the master scheduling process considers families of parts. These are groups of items that are similar enough that they can be treated as one unit for planning purposes. When you disaggregate a Level 0 family, you separate the forecasts for the Level 1 parts in the family using the percentages established for the planning structure. A forecast for a Level 0 family can be a sales plan or another predicted value.
A Level 0 part is a blow-through part that cannot be manufactured. A Level 1 part can actually be built, stocked, and shipped to customers. During disaggregation, the forecast for a Level 0 family is transferred to the Level 1 parts within it.