So I search terms was "atan2 in layman's terms and this is the main result(not a link but a paragraph on its own):

What does atan2 return?

atan2() method returns a numeric value between -π and π representing the angle theta of an (x, y) point. This is the counterclockwise angle, measured in radians, between the positive X axis, and the point (x, y)

Go further into an article linked below it and I came across;

Description

The Math.atan2() method returns a numeric value between -π and π representing the angle theta of an (x, y) point. This is the counterclockwise angle, measured in radians, between the positive X axis, and the point (x, y). Note that the arguments to this function pass the y-coordinate first and the x-coordinate second.

So there is confirmation of the order, in fact literally every result I read into said the same about the order. Couldn't really find out/work out why. Have no clue what is meant by between negative/positive pi so i am assuming that is something used under the hood so to speak.

Also worthy of note is that the angle in the pic is counter clockwise which I have also read a number of times in various articles. I keep reading that atan is useful for working out the arctangent of only 2 quadrants(1 and 4) whereas atan2 is useful for all 4(ie on the pic you can see the divisions making 4 quadrants). What the heck all this means is well, I do not know! I mean what the purpose of each is, is what is missing - ie how to choose which you would use, when you would use and why...so atanfull I found useful in DBP when working out how to rotate a tree to match its billboard based on the camera and then slowly rotate it once the billboard is no longer needed towards a fixed angle(search my thread history and choose Tree LOD as the code is there). I can only guess that atan and atan2 would be useful for performance reasons maybe?

Win 7 Pro 64 bit SP1, AMD A4-5300 APU 3.4GHz, 8GB DDR3, NVidia GeForce GTX 750 1GB GDDR5, ASUS A55BM-E