A primary cost elements is for CO the equivalent as accounts in FI with the same codification that is detailed in the page GL Accounts codification tables.
In PF2
The codification of a primary cost element is:
6 when it is a cost & 7 a revenue
9 digits : it can depend on the use of the cost element
In WP2
The codification of a primary cost element is:
98 : primary cost element
6 digits that can depend on the use of the cost element