Payments of actual social contributions may be compulsory by virtue of a statute or regulation, collective agreement in a particular industry, agreements between an employer and employees in a particular enterprise, included in a contract of employment or, in certain cases, contributions on a voluntary basis.
The voluntary contributions referred to here cover:

• social contributions which persons who are not, or who are no longer, legally obliged to contribute or continue to contribute to a social security fund;
• social contributions paid to insurance enterprises (or health insurance and pension funds classified in the same sector) as part of supplementary insurance schemes organised by enterprises for the benefit of their employees and who join voluntarily;
• social contributions paid to health insurance institutions open to employees or self-employed persons.

To distinguish between social contributions which are compulsory and those which are voluntary, a supplementary level is included in the classification. The following subcategories are defined as follows:

• compulsory employers’ actual social contributions (D.61111);
• voluntary employers’ actual social contributions (D.61112);
• compulsory employees’ social contributions (D.61121);
• voluntary employees’ social contributions (D.61122);
• compulsory social contributions by self and non-employed persons (D.61131);
• voluntary social contributions by self and non-employed persons (D.61132);