If you select the field Transfer auxiliary cost component split, the system transfers this split when it reads a cost estimate from Product Cost Planning.
In Product Cost Planning, there are two types of cost component split -- the cost component split for cost of goods manufactured and the primary cost component split.
You can store both of these cost component splits in parallel in Product Cost Planning. If you do so, you need to define one of these as the main cost component split and the other as the auxiliary cost component split. You also have the option there of storing only one of the cost component splits.
Which of these is the main cost component split is defined in Customizing for Product Cost Controlling in the settings for the costing variant . These settings are valid per company code and plant.
View: Assignment: Organiz. Units - Cost Components Structure
View: Cost Component Structure