What is the implied plane of '2D' functions?

I was just looking at FVector.Size2D() but there is no note in the documentation that states which plane is assumed for it? I’ve noticed other functions etc that state it’s the 2D version of the function with no documented plane. Is it XY?


2D functions that returns scalars. like Size2D(), use the X and Y coordinates. 2D functions that return vectors, like GetClampedToMaxSize2D(), modify the X and Y coordinates and keep the Z coordinate of the input.

See the source.