it is the pixel count.
1200x700 simply means: 1200 pixels in width and 700 pixels in height
so 1920x480 would not choose one of the other resolutions, it would stretch the video
the 360p here actually stands for the height. With this value and the screen ratio you can calculate the real resulotion.
so lets asume you use a typical 16:9 screen:
- 360p = 640×360
(360/9*16 = 640) - 720p = 1280×720
(720/9*16 = 1280) - 1080p = 1920×1080
(1080/9*16 = 1920) - and so on..
the resolution youtube uses maybe is a bit different (e.g. the video player could be 5 pixels smaller due to the way they programmed it) and the developer just made it easy by writing the common value, so that everybody would understand it