The method in which in-context parts are created as taught in the SolidWorks training classes is as follows:
Adding new parts into an assembly
When you create a new part in an assembly, the part is given a default name and you select a plane (or planar face). The name is used as the temporary part name while the selected plane orients the Front reference plane of the new part. An InPlace mate is added to the assembly to maintain the position of the new part. The location of the origin of this new part is determined by projecting the assembly origin onto the Front plane of the new part.
Building parts in an assembly
As the new part is created, the selected plane face becomes the active sketch and the part is in Edit Part mode. The part is created using standard methods and references to other geometry in the assembly.
Creating in-context features
When you reference geometry that is in another part while creating a feature, you are creating what is called an in-context feature. For example, referencing the edge of a shaft when making its mating hole in another part creates a relationship between the shaft and the hole. A change to the diameter of the shaft would cause a corresponding change to the diameter of the hole.