In the Frog example, a reader invokes targets, such as build, that are not listed in the author's application makefile. These targets are supplied by our laboratory's ReDoc rules and are merely included in the document's makefile (Doc.defs.top, Doc.rules.red, Doc.rules.idoc). They ensure a consistent reader interface, prevent the author from re-implementing the ReDoc rules in every makefile, and accumulate the wisdom of the entire community. They are formulated in a way that any individual author can override them. Experience shows, however, that overriding is hardly ever necessary or desirable.
An author or reader does not need to know the implementation details of the ReDoc rules to use the rules (most researchers at our laboratory have never inspected the ReDoc rules). But you may wonder how the rules operate and how you may have to adapt the rules for your community's computational environment.