The first broad distinction is between overt and less overt borrowing. An overt case carries an earlier meaning or verbal construction in a form that makes the relationship comparatively recognizable.
In less obvious cases, alteration of wording, presentation or conceptual perspective obscures direct correspondence. Their fuller treatment follows later; this lesson establishes the basic idea of overt taking.
The distinction would be lost if similarity in wording, arrangement and meaning were treated as one and the same matter.