When an aircraft uses air to air refuelling are there constraints on the maximum altitude this can take place at?
For instance I'm guessing that a Boeing Globemaster when on a long journey would cruise at 8,000m or higher (Wikipedia says service ceiling is 14,000m / 45,000'). Is there any reason why air-to-air refuelling can't take place at that sort of height?
I'm wondering whether there's something about the lack of air pressure that makes it problematic to transfer fuel or the speeds that must be used for the transfer are not viable at very high altitudes.