Self-employed persons
A self-employed person exercises a professional activity for his own account and is therefore not bound by an employment contract or a status. He is not an employee or civil servant.
Examples: farmers, traders, physicians, company administrators, working partners, etc..
Exceptions

Assistants
An assistant assists a self-employed person or replaces him during the exercise of his profession. An assistant is not bound by an employment contract and is not necessarily a relative of the self-employed person.
Exceptions

Helping spouses
The husband/wife or life partner really helping the self-employed person in the business, has to join the social security fund of the self-employed person he/she helps.

Companies
Companies that fall under the Belgian company tax, must join a social security fund and must pay an annual contribution. However, this rule does not apply to starting companies.

Mandataries and partners
Attention! Also unpaid mandataries and working partners must join a social security fund.
Silent partners who only bring in capital and unpaid mandataries entitled to a pension on the other hand, are not required to do so because they are not required to pay insurance.