Assemblies. Freecad technically has the ability to create assemblies, but it's been in beta for years, and you have to compile it from source in order to get it.

Also, I find solvespace much easier to use the Freecad. Freecad has such a myriad of options and controls that I find myself often taking a while to find the one I'm looking for. Solvespace is a much more focused tool, having fewer but easier to find controls.