The form interpolant is a way of estimating the orientations within a set of structural data (dip and dip direction). Rather than interpolating a single value like in the regular RBF interpolant, the dip and dip direction are interpolated. The predicted orientations are represented by several isosurfaces.
The form interpolants can be used to visualize large quantities of structural data to better understand the geology. The form interpolants can be evaluated onto other surfaces to visualize changes in a fabric across the model boundary. These can also be used to as structural trend inputs. Rather than manually digitizing a mesh where you feel is the best fit for the trend, the form interpolant can do this.
The 0.0 isosurface is created off the centre of the bounding box and is the shape of the interpolated data. The other surfaces (5 by default) are offset from there and are generated based on distance within the boundary. The offset in each direction is indicated by positive and negative surfaces. Rather than true distance, they are normalized, so you will see values such as 0.5, -0.5, 1 etc. The reason for the normalization is so that the isosurfaces are consistent no matter what the input data (i.e. a -0.5 isosurface will be approximately the same distance away from the 0.0 isosurface, even in datasets where the bounding box is either much smaller or larger). The normalization is such that 1, -1 will generally be just within the edges of the bounding box. There are some geometries where the numbers may need to be extended to 3, -3 to include the outer isosurfaces, depending on the data.