[en] The conformal reference systems (RS) are determined on the basis of infinitesimal drag. The conditions for the values describing BS are obtained under the assumption that the corresponding monad vector and the conformal Killing vector are proportional. These conditions, which represent a set of differential equations for the components of the monad field, are met if RS is conformal. The conditions for intersection of the sets of conformal and harmonic RS are considered. It is proposed that RS are called the Killing RS if the monad vector is proportional to the Killing vector. It is noted in conclusion that all the Killing RS are harmonic (but not vice versa) and stationary