The term instance has two meanings in 3ds Max. One is the standard definition; the other is specific to clips in the Motion Mixer.
In general, an instance is a completely interchangeable clone of the original object. Modifying an instanced object is the same as modifying the original.
In the Motion Mixer, when the same clip is used more than once on tracks, the clip versions are either instances or adaptations of each other.
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.
For example, suppose you scene contains two bipeds that are exactly the same size, and you use the same clip in both bipeds' mixes. The clips within one biped's mix are instances of one another, and clips within the mixes of the two same-sized bipeds are also instances of one another. Instances have the same number appended to the ends of their clip names in the Mixer.
Suppose you then add a third biped of a different size, and use the same clip in that biped's mix. The new version of the clip is an adaptation of the clip used on the first two bipeds. An incremental number is added to the end of the clip name in the Mixer.
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, but no distinction is made between instances and adaptations at that point because the clip appears only once .
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. If the change is the same, 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.