Dear Milad Sakkaki ,

To finalize your geometry you have to use one of these options. Form Union is the default option which is suitable for most cases, but form assembly is an alternative choice for specific cases, such as rotating machinery interfaces, contact simulations with touching objects, in the cases you need more effective meshing like in complex PCB and chip design and etc.

Here's a useful explanation on these two features on the linke below:

https://www.comsol.com/support/knowledgebase/1216/

Dear Milad Sakkaki ,

To finalize your geometry you have to use one of these options. Form Union is the default option which is suitable for most cases, but form assembly is an alternative choice for specific cases, such as rotating machinery interfaces, contact simulations with touching objects, in the cases you need more effective meshing like in complex PCB and chip design and etc.

Here's a useful explanation on these two features on the linke below:

https://www.comsol.com/support/knowledgebase/1216/

Please read carefully the documentation on Entities and Objects in Version 4 of COMSOL , it is too long to write it all here. Mostly the use of "union" assembly is for contact and specific user physics, but there are far more behind. Take the time its really worth it to understand what Entities are for, and how they are created from the geometry. There are more in the others documents too (use an indexer on your COMSOL pdf doc to quickly search inside them). you also can find a very detailed explanation just typing "The Finalize Node—Finalizing the Geometry" in the COMSOL Documentation: once you have opened COMSOL, click on the icon with the interrogative point. Or you can equally go to Help > Documentation. Or you can just click Ctrl+F1. I think that explanation is identical to that mentioned by Ivar!. In practice, you can first form a union of the two parts, then, when you finalize your geometry, you click "form an assembly" which will assemble the remaining parts. The union can specify to keep the internal boundaries which has the same affect as a "form a union". A mesh of the union parts will match on the boundaries, whereas, the mesh of the assembly parts can have un-matched mesh if you desire and specify that way.

I hope it will be helpful for you.

All the best.