Diff between "join" and boolean "union"?

 From:  Michael Gibson
1689.2 In reply to 1689.1 
Hi angleiclight - use Join if you have surfaces that are touching edge to edge, like this:

Above I modeled 2 surfaces in separate steps by extruding those 2 different curves.

That has created 2 surfaces that are independent objects, but they are adjacent to one another and have an overlapping edge.

You can use Join to glue together 2 surfaces that are separate but adjacent like this into one single connected object. Sometimes this can be used to do detailed tweaking to a couple of surfaces of a solid - you can use Edit/Separate to break out some surfaces from a solid, edit them by trimming them/transforming/etc.. then join the edited surfaces back with the others to make a solid again.

Join is also used to glue together 2 adjacent but touching curve segments into a longer curve object.

The booleans are meant to be used when objects actually push through each other so that they need to be intersected with each other to get the result.

Like for example here are 2 solids:

If you arrange these so that they are punching through each other like this:

Then you would use boolean union to fuse them together into a single object and discard the interior parts:

That does not involve just gluing existing edges together, that involves calculating the intersections between the 2 objects and creating new edges where the solids/surfaces intersected, for example a new edge is shown highlighted above.

So Join is for gluing edges for surfaces that are already touching edge-to-edge, and booleans are for fusing objects together that need cutting and intersecting to happen.

Also another way to say it is that Join does not discard material, while booleans are focused on discarding some part of the objects involved, like above they discarded the parts of the sphere that were inside the cylinder, and discarded the parts of the cylinder that were inside the sphere.

- Michael