When you create a cost estimate with quantity structure, the system builds a quantity structure from the PP master data (that is, BOMs, routings, or master recipes) to determine the materials and internal activities required to manufacture the product.  

The quantity structure is created automatically using the following data:

• BOM and routing (production by lot size).
• BOM and rate routing (repetitive manufacturing).
• Quantity Structure Control Through the Initial Screen of the Cost Estimate

Costing copies this quantity structure automatically to costing items of type M (material) and E (internal activity). 

You can control how the the quantity structure is built through:

• Customizing.
• Material Master Record.
• Initial Screen of the Cost Estimate.

The quantity structure control is a process by which the system searches for alternatives, should multiple BOMs and/or routings exist for a material.You enter the quantity structure control ID in the costing variant. This ID describes a list of priorities for the BOM and routing through the BOM application and routing selection keys. 

The BOM application controls the following:

• The order of priority of the BOM usages (selection ID).
• The priority of an alternative BOM for a specific multiple BOM.
• Whether the system includes only those BOMs with a status containing particular status indicators

The routing selection ID determines how the system selects a routing. You can define several priorities. You assign selection criteria (task list type, task list usage, and task list status) to each of these priorities. The routing that corresponds to the selection criteria with the highest selection priority is selected. If, however, no alternative routing can be found, the system continues searching using the selection criteria of the next selection priority.

When determining the BOM and routing, the system also checks the following:

• Whether the BOM and the routing are valid on the quantity structure date.
• Whether the lot size in the BOM and in the routing are the same as the costing lot size.

Related the quantity structure selection please consider the following documents:

458547 - CK40N: - SAPSQL_ARRAY_INSERT_DUPREC in SAPRCK62

1832841 - CK11N picks incorrect alternative BoM

420251 - Fixed costs proration in costing

1962696 - Cost estimate: Runtime error MESSAGE_TYPE_X CO 889 in program SAPLCOSD