Indications are an essential tool of clinical practice. Indications are to be based on rules. Clinical practice guidelines (GCPs) are compendia of indication rules (IRs) with an evidence base as strong as possible. Further clinically relevant indication rules (IRs) can be found outside guidelines and again not all are supported by stronger evidence. The text describes and assesses seven classes of IRs based on only weak evidence. To do so, a sufficiently selective operational definition of indication (rule) and evidence had to be developed.