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From a page move: This is a redirect from a page that has been moved (renamed). This page was kept as a redirect to avoid breaking links, both internal and external, that may have been made to the old page name.
From a monotypic taxon: This is a redirect from a monotypic taxon to its only lower-ranking member. In a biology-related article, when for example a family has only one genus, the family may be a redirect to the genus.
Use {{R to monotypic taxon}} when making a redirect to a monotypic taxon from its only member, for example, from a species to the genus of which it is the only member.