It is said that this node outputs a V2, but I found this “V2” is not as the same as the V2 I created by clicking 2 button and pressing LMB. The former V2 is gradient , but the later is pure color
It’s not just a “v2”.
It’s the V2 value of every UV face.
0 to 1 on U
0 to 1 on V
With 0,0 being the top left corner, and 1,1 being the bottom right corner.
What those value actually are obviously depends on the position on the UV that you are looking at.