A three dimensional surface is mathematically described by a two-dimensional parameter space. That is, every three dimensional location on a surface is defined by an equation that have only two variables, a U parameter and a V parameter. The natural domain of a surface is defined by U-V parameter pairs, with each parameter having a value range of 0.0 to 1.0. Each value pair maps to a unique 3D location on the surface.

An Isoline is parameter space is defined by constant parameter for one of the parameters. For example, every point on the surface that has a U value of 0.5 defines an Isoline in the two-dimensional parameter space. For a regularly parameterized surface with basically a rectangular natural boundary, this isoline maps to a 3 dimensional curve on the surface that would roughly bisect it. For more extreme surface shapes with more irregular parameterizations, the physical 3D curve may not be that close to bisecting the actual 3D surface.

In IDD, you can create UV curves on surfaces by defining an isoline. The isoline is defined by selecting a point on the surface, and a parameter direction. The point on the surface defines the U and V parameter values, and the parameter direction defines which of these values are used as the constant value to define the UV isoline to be mapped to the 3D surface.