A while ago on a flight into EDI when I had recently got my IMC and had very little IFR experience in CAS I had to ask Scottish control what this meant after he had asked me my heading and said "make that your radar heading". He patiently explained that it was the heading I now had to maintain under the RCS until I was told otherwise. Since then I have found this phrase used very often in CAS but am still surprised that there is no mention of it in CAP 413.