Related Rule
Practice Relating to Rule 31. Humanitarian Relief Personnel
Sweden’s IHL Manual (1991) states that the rules on the recognition of the role of aid organizations under Article 17 of the 1977 Additional Protocol I and on the protection of personnel in relief actions under Article 71(2) of the 1977 Additional Protocol I have the status of customary international law. 
Sweden, International Humanitarian Law in Armed Conflict, with reference to the Swedish Total Defence System, Swedish Ministry of Defence, January 1991, Section 2.2.3, pp. 18–19.