What is the eugenic "indication" in Casti Connubii?

In Pope Pius XI's encyclical *Casti Connubii* , we read: > What is asserted in favor of the social and eugenic "indication" may and must be accepted, provided lawful and upright methods are employed within the proper limits Within the context of paragraph 66, the "indication" (*eugenicæ* indicationis ) is condemned when exploited "for the killing of the innocent", e.g., abortion (cf. paragraph 63). But in the larger context, * What is eugenic "indication"? Presumably it is related to "hereditary transmission" of genetic traits (cf. paragraph 68). * What is a lawful employment of this eugenic "indication"?