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Welcome to the CPL Air Law question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
While HF transmissions have the advantage of long range communication beyond 'line of sight' they are also affected by sun spot and ionospheric disturbances. This can result in particular HF frequencies being more or less effective as ionospheric conditions fluctuate. Because of the random nature of these disturbances, pilots are advised which frequencies are working best at a given time and in a given area. The 'primary HF frequency' can be obtained from briefing or passed to the pilot while he/she is still in VHF range. I don't think you will find a specific reference to this in the commonly available documents. Maybe someone can suggest one.