NEC ECC: Retention in Option F

Under option F, what is the retention (clause 24) in Defined Cost referring to? The concept of withholding is similar as retention?

Retention is withheld from a Contractor if X16 retention has been chosen by the Employer.

The clause you highlight in option F is talking about Defined Cost being any payment to Subcontractors, without taking into account amounts deducted for retention within their contract with the Contractor if they have also used X16 with their supply chain.