.For some types of editing operations, you need to specify that the changes you are making belong to the sub-assembly, and not to the top-level assembly. This includes any case where the edits could be successfully applied to either one. For example, you can add an assembly feature hole either to a sub-assembly, or to the top-level assembly, with different results.
You must activate the sub-assembly to perform these editing operations:
Add a component into the sub-assembly. You cannot add a new component in the context of a sub-assembly.
Delete a component from a sub-assembly.
Add a component pattern, an assembly feature cut or hole, a sketch, or reference geometry to the sub-assembly.
Use dynamic assembly motion on sub-assembly components. You can fix, float, and move sub-assembly components when the sub-assembly is active, to visualize the movement within the sub-assembly.
Add a mate between components of the active sub-assembly (both at the same level in the hierarchy, or one at a lower level, in a nested sub-assembly of the active sub-assembly).
NOTE: While a sub-assembly
is active, the mates in the top-level assembly that position the sub-assembly
with respect to the top-level are temporarily ignored. Top-level mates
are solved again when you return to editing the top-level assembly and
click Rebuild .
You cannot add a mate between a component within an active sub-assembly and a component outside the active sub-assembly.
To activate a sub-assembly for editing in context:
Select the sub-assembly and click Edit
Component 
on the Assembly toolbar, or right-click
the sub-assembly icon in the FeatureManager design tree, and select Edit Sub-assembly.
When you activate a sub-assembly, it turns blue, and the rest of the assembly turns gray, just as it does when you edit a part. When you edit a component of a sub-assembly, only the individual part being edited turns blue. See Colors When Editing a Component.
To return the editing focus to the top-level assembly:
Click Edit Component again,
or right-click and select Edit Assembly.