Join

Join does not change the surfaces. It is merely a test to see if the surfaces edges are within 2x tolerance. If they are, then they are tagged as joined and not available for further joining.
When a curve is selected first or using sub-object selection, surface edges or mesh edges can be joined with the curve to create a polyline or polycurve.
You can join curves that touch end-to-end.
The order the objects are selected determines the layer of the new joined object.
You can join surfaces and polysurfaces that touch at naked edges.
You can join meshes that do not touch (disjoint meshes).
Joining does not change the underlying surface geometry. It simply "glues" adjacent objects together so meshing, Boolean operations, and intersections can cross seams without gaps.
To change a surface's geometry so it fills in a gap, use MatchSrf or fill the gap with a new surface created by FilletSrf, BlendSrf, BlendEdge, FilletEdge, NetworkSrf, or Patch.
To change two adjacent surfaces into a single surface, use MergeSrf. Pay special attention to the setting of the Smooth option to get the geometry you want.

Select the objects (curves, surfaces, polysurfaces, or meshes) to join.
Use SelChain to select a string of curves that touch end to end.

Select an object (curve, surface, polysurface, or mesh).
Select the next object.
To select a surface edge as a curve to join, see sub-object selection.
When you are finished selecting objects to join, press Enter.

Command-line options

The Undo option removes the last selected object from the join operation.

The JoinDisjointMeshes option allows joining meshes that do not touch into one object.

Hidden command-line options

Selects connected curves or edges based on continuity conditions.

See also