English speaking or comprehension.
Although a modest PPL I have had Scottish and Geordie A/T where I have had to "say again" several times. It is the ability of the pilot to comprehend the instruction that is crucial and you only have to listen to some of the controllers in the US to know that to English ears their speech is almost indecipherable sometimes.
When you have requested information and are awaiting or expecting a reply comprehension is high but "out of the blue" queries or instructions can be extremely difficult to comprehend first time.