Building or editing components inside an assembly is a powerful tool both in terms of creating intelligent feature associations and visualizing your design. However, editing a part in an assembly often leads to the creation of external references from the part to the assembly. This means that certain features or sketches in the part are dependent on the assembly. In many cases, this is beneficial. However, in other cases a user might want to ensure that the part being edited in the context of the assembly does not have any references to the assembly.
If you wish to work within an assembly but avoid creating references between components, go to Tools>Options>External References and check the box Do not create references external to the model. You can also do this by selecting the No External References button on your default assembly toolbar for a temporary cession of this function.