CPF官方网上说：
CPF contributions are not mandatory for Singapore citizens or Singapore Permanent Residents who work overseas. If you wish to continue making CPF contributions for your existing employees who are posted overseas, these are deemed as voluntary contributions. You have to register for a new CPF Submission Number (CSN) for such payments.
IRAS官方网上说：
If you work overseas, it is not mandatory for you and your employer to pay CPF contributions.
However, you may wish to build up your retirement savings by making voluntary CPF contributions, up to the approved limit. However, if your employer in Singapore is paying CPF contributions for you, the employer's contributions will count towards the limit.
Do note that voluntary contributions do not qualify for tax deduction.
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