The terms mandatory law (ius cogens) and non-mandatory law (ius dispositivum) refer to the relationship between a rule of private law and the agreement by the parties. While non-mandatory rules can be set aside by contracual agreement (and sometimes also by a unilateral act of one party), mandatory rules do not allow for derogation by the parties. Since party autonomy (freedom of contract), i.e. the freedom of individuals to determine their legal relationships according to their own discretion, is the most fundamental principle in any liberal system of private law, non-mandatory provisions of law are the general rule whereas mandatory provisions are the exception (cf 2.(b), 3.(b) below).
Grigoleit, Hans Christoph, Mandatory Law: Fundamental Principles (June 30, 2011). MAX PLANCK ENCYCLOPAEDIA OF EUROPEAN PRIVATE LAW, Jurgen Basedow, Klaus J. Hopt and Reinhard Zimmermann, eds., Oxford University Pres, December 2011.