1D when it comes to a location on a surface or a curve. If a 3D point shares a location on a surface we can represent it by means of the U and V co-ordinates of that surface.
In your example above the 4 surface corners are {2,2.5,0}, {17, 2.5, 0}, {17, 19, 0} and {2,19,0}. Unless you reparametrise the domains they will typically take the same domains as the curves that constructed them in this particular case the lengths (but these curves are only that length at the edges and only when you created the surface).
So the U domain is 0 to 15 (17-2) and the V domain is 0 to 16.5 (19-2.5). Even if you transformed the surface to another location or another shape these domains will not change and therefore the UV co-ordinate will not change. If you reparemterise the surface then the domains are set to 0 and 1 in both directions and this might be easier to work with. You can think of them as a percentage then, a UV location of {0.5, 0.5} of a reparameterised surface will always be in the middle of the 2D space.
All points on a surface in 2D have a 3D space co-ordinate as well, but not all 3D points have a 2D co-ordinate. This is why we need to use the Surface CP to get a UV value to evaluate a surface at a given point.
Incidently the 1D co-ordinate of a curve is represented by the parameter t
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