Hi stinkie,

> Or more like, say, two planes that have a common edge?

Yes - like that.

Like for example say you create a shape by sweeping some curves that are not closed, that creates a surface instead of a volume, like this:



It's sort of like a sheet.

Now say I create a second surface sheet adjacent to that, for example by extrusion:



These are now 2 separate objects (that's what I mean by "individual" surfaces). This is the kind of situation where you do need to use Join to glue them together into a single object that has a shared edge in it - that way that shared edge will get meshed without any holes or cracks appearing along the edge. That applies to MoI and Rhino both.

- Michael