CSN is the number you use to transact with CPF Board e.g. making CPF payments, paying foreign worker levies and making any request relating to employer matters.
For entities, the CSN is the “Unique Entity Number (UEN) + CPF Payment Code”.
For employers trading under their own names (e.g. individuals hiring a personal driver, gardener, local domestic worker etc), the CSN is the “NRIC/Foreign Identification Number (FIN) + CPF Payment Code”.
For individuals hiring foreign domestic workers or the self-employed, the CSN is the individual's NRIC/FIN.
source : www.cpf.gov.sg