In the Motion Mixer, when the same clip is used more than once on tracks, the clip versions are either instances or adaptations of one another.
The same clip used more than once for one biped, or for different bipeds of the same size, is an instance. The same clip used for different-sized bipeds is an adaptation.
These terms are used because the Mixer adapts each loaded clip to the biped's size. The first time a clip is loaded, the Mixer adapts the clip as needed. When the clip is cloned or loaded again, the Mixer adapts the new clip to the biped as needed, then compares the change to previously loaded versions to see if it's the same. If so, the new clip and its previous versions are instances of one another. If not, the new clip and previous versions are adaptations of one another.
In footstep animation, the term adaptation refers to keys generated for a footstep sequence. When you edit active footsteps, body and leg keys are adapted automatically. By analogy, the footsteps become a kind of "gizmo" for manipulating the keyframes of your character's animation. In most cases, edits you make to footsteps will act upon your keys in an intuitive fashion.