Another term for opting-in in English is 'working agreements'.

There are three different ways you can work as a sex worker in the Netherlands.
If you don’t work for a sex business owner, but from your home for example, then you are self-employed.
If you work in a sex club, massage parlour, private home or in the escort service, you work for an employer. In that case, you may either work as an employee or under the working agreements. Working agreements (or opting-in) is somewhere in between being self-employed and an employee.

If the working agreements apply to you, this means you always work for a sex business owner and you are not considered as working in paid employment. Your employer, who has an agreement with the Tax Administration, deducts the taxes and contributions from your income and pays them to the Tax Administration and the Employee Insurance Agency (UWV), the organisation that regulates social aloowances.
This means you don’t have to do this yourself.

Talk to your employer about what applies at his or her company: paid employment or working terms.
Read the following pages for more information about your rights and obligations when you work according to the working agreements.