Additional Wage (AW) is computed per employer per year. The limit will "refresh" when they join a new company in the same year. You can refer to: https://www.cpf.gov.sg/Assets/employers/Documents/ExamplesonAdditionalWageCeilingComputation.pdf.
Here's an example of how it works:
In 2017, Employee A hits total ceiling (Ordinary Wage (OW) + AW) of $102,000 in Company A by Sep 2017. Then he left the company.
In Oct 2017, he joined Company B. His AW ceiling for Company B in 2017
= [$102,000 - (OW ceiling from Oct to Dec 2017)].