There are 6 categories of message, and air traffic control will prioritise them in strict order.
Priority one is distress or "mayday" aircraft where imminent assistance is required. Priority two is urgency or "pan pan" aircraft where immediate assistance is not required. Priority three are communications relating to direction finding (think "lost") - an aircraft may be asking for a fix, QDMQDM —Magnetic bearing from an aircraft to a beacon or station, QDRQDR —Magnetic bearing from a beacon or station to an aircraft or other such Q code. Priority four are flight safety messages - this is 99% of what you will have learned in comms, including clearances to land, take off, climb, descend, taxi instructions, hold instructions, speed instructions, altitude instructions and much more besides. Note that weather information of immediate relevance to an aircraft (such as wind shear) is also included here in priority four. Priority five are meteorological message not of immediate concern. Priority six are flight regularity messages - requests for ATCATC —Air Traffic Control to pass information to your operator, for example, if you do not have a company frequency.
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