From Longman Business Dictionary
obligatoryo·blig·a·to·ry /əˈblɪgətəri-tɔːri/ adjectiveadj formalLAW something that is obligatory must be done because of a law, rule etcSYN COMPULSORY, MANDATORYThe Housing and Town Planning Act made it obligatory for local authorities to prepare surveys of their housing needs.