The invention relates to a medical instrument, comprising a signal transmitter and at least one activating device for activating the transmitter which is provided on a transmitter unit which is separate from the instrument, wherein the transmitter unit is a unit which can be fastened to the exterior of the instrument. It also relates to a method for controlling a treatment-assisting medical software by means of a signal transmitter which is arranged on a medical instrument, wherein the instrument is positionally detected by means of a medical tracking system, and wherein the software assigns a different significance to the signal output by the transmitter depending on the position of the instrument.