Related Rule
Practice Relating to Rule 21. Target Selection
Sweden’s IHL Manual (1991) considers that:
In certain circumstances it is possible to reduce the risk to the civilian population and to civilian property if the military commander selects a different objective, from which he can achieve about the same military advantage as from the prime objective. In many situations, however, it is impossible to denote an alternative objective, for which reason the rule concerning second-line objectives has been given the reservation mentioned by way of introduction: “when a choice is possible”. 
Sweden, International Humanitarian Law in Armed Conflict, with reference to the Swedish Total Defence System, Swedish Ministry of Defence, January 1991, Section, p. 72.