Let's say you open a cat process.  And that is the ONLY window you have open.  Now you go into the main level MFG product by double clicking it.  Now you are in the Assembly Design Module.  Let's say you move things around a bit.  Now these parts are only moved around in the Process.  If you have different constraints in the Product (opening that product alone) the changes won't be there. When you are working with a process, the product is just an INSTANCE of the product.

So...the only reason to ever keep an assembly flexible is if you don't want to move the parts around in the product EXCEPT for manufacturing purposes.  THIS IS BAD PRACTICE.   BECAUSE THE MFG PRODUCT SHOULD ALWAYS BE A DEDICATED PRODUCT ONLY FOR MFG.

SO....always keep everything RIGID.  ***Note: the default is Flexible!***

This way, any changes you make to the product whether in a seperate window or by double clicking it while in MFG module will also be reflected immediately in the process.

Flexible is really just for people that are in too much of a rush to make a new product dedicated to MFG.  Flexible will cause MAJOR problems.  Plus, my computer works MUCH faster if I make edits with my product or just a subproduct open in another window.   Less crashes, and less confusion.

Hope this helps explain.  Go rigid, or go home...that's what SHE SAID!!!