“A secondary cost element represents postings that occur between controlling objects within CO. A secondary cost element is essentially an account that exists only in the CO module, not in FI. When costs are moved (e.g., from one cost center to another using an assessment, or from a cost center to a production order), no postings are made to the FI P&L. Instead, a secondary cost element is used to track that posting in a CO document.” – taken from here https://blogs.sap.com/2010/12/20/back-to-basics-secondary-cost-elements-101/