it's the x-z axis...
..and that doesn't explain why the y coordinate is listed first in the data tag and in commands either.
But the current x_rotation isn't related at all to the x axis...
Even if this is intended, it should be changed for these reasons:
•horizontal rotation is entered first everywhere else (nbt, teleports, etc) and should therefore be the x-coordinate
• since then horizontal/vertical rotation would correspond to x/y relative to a player's crosshair, you could simplify local coords (^x ^y ^d instead of ^l ^u ^d)
•it would be a lot less confusing to everyone learning commands
this should just be a simple fix, please take care of it soon so I can finish the project in this test world (it would probably be a good realms minigame)
Yet minecraft's y axis is the vertical axis instead z...