This dialog lets you choose the way to join two surfaces.
This tab chooses the zip algorithm. Zipping concatenates the CV lattices of the two original surfaces. Zipping can change the shape of the original surfaces, but compared to joining it usually produces a simpler surface that is easier to edit.
By default, the ZIP tab is active.
If both curves are untrimmed point surfaces, the result of zipping is a point surface. In all other cases, the result is a CV surface.
This tab chooses the join algorithm. Joining first creates a blend surface between the two original surfaces, and then makes all three into a single surface. Joining does not change the shape of the two original surfaces.
If both surfaces are point surfaces, the result is a point surface. If one or both surfaces are CV surfaces, the result is a CV surface.
A distance in 3ds Max units. If the gap between the surfaces you are joining is greater than this value, the join is created by first creating a blend surface and then joining the three parts. If the gap is less than this value, or if the surfaces are overlapping or coincident, 3ds Max doesn't create the blend.
Creating a blend and then joining the three surfaces into a single surface is the better technique. The result matches the parent surfaces well. Without the blend step, the resulting surface can deviate from the parent surfaces, in order to maintain smoothness. (The amount of deviation depends on how far from tangent the two input surfaces were at the join.)
A problem arises when the gap is too small. In this case, 3ds Max generates the blend but because there isn't enough room for it, the resulting surface has a loop. To avoid having this loop, set the Tolerance higher than the gap distance.
If you set the tolerance to 0.0, 3ds Max chooses a value to use for the Tolerance.
Modify panel 