Build in Place - by default, the 'Create Assemblies' command will add all hosted details to an Assembly. But if you add the 'Build in Place' parameter to hosted elements, and turn it ON, they will not be added to the Assembly. It's useful when you have a few nested elements and some of them should be installed on site, so you don't want to have them in the Assembly.
Parameter should be created like this:
It's a Yes/No parameter, and its name should be exactly that, so the software can recognize it.
Here's a Family with some nested Families, some of which have the Build in Place parameter.
So, in the model all hardware is in place...
...but in the Assembly loose plates are excluded: