Payments of actual social contributions may be compulsory by virtue of a statute or regulation, or they may be paid as a result of collective agreements in a particular industry or agreements between employer and employees in a particular enterprise, or because they are written into the contract of employment itself. In certain cases, the contributions may be voluntary. The voluntary contributions referred to here cover:

(1) social contributions which persons who are not, or who are no longer, legally obliged to contribute pay or continue to pay to a social security fund;

(2) social contributions paid to insurance companies (or friendly societies and pension funds classified in the same sector) as part of supplementary insurance schemes organised by enterprises for the benefit of their employees and which the latter join voluntarily;

(3) contributions to friendly societies with membership open to employees or self-employed workers.

To distinguish between social contributions that are compulsory and those that are not, a supplementary level is introduced in the classification:

(a) compulsory employers’ actual social contributions (D.61111);
(b) voluntary employers’ actual social contributions (D.61112);
(c) compulsory employees’ social contributions (D.61121);
(d) voluntary employees’ social contributions (D.61122);
(e) compulsory social contributions by self and non-employed persons (D.61131);
(f) voluntary social contributions by self and non-employed persons (D.61132).