Most top-down relationships are created while editing a part within an assembly. This is also referred to as in-context editing because you create or edit the feature in the context of the assembly, instead of in isolation, as you traditionally create parts. In-context editing lets you see the part in its correct location in the assembly while you are creating the new feature. Additionally, you can use geometry of the surrounding parts to define the size or shape of the new feature.
Even if you are not using top-down design methods, you can still edit a part in the assembly window for convenience. For example, while working in an assembly, you could edit a component's extrusion to reverse its direction or edit a fillet to add more edges from the same component. These are not considered top-down design because they don't use any of the other parts in the assembly.
When you edit a part in the context of an assembly, you can use colors to indicate which component is being edited. You can also change the transparency of assembly components while editing a part.
To edit a part while in an assembly: