The Reservoir is a storage area for clips (BIP file) used by the Motion Mixer. The Reservoir contains a single entry for each clip adaptation used in the Mixer. You can use the Reservoir to load previously used clips, save adaptations to new BIP files, or view a clip's adaptation data. See Using the Reservoir.
The purpose of the Reservoir is to save memory. Instead of storing the animation and adaptation data for each clip inside the clip, it is stored in the Reservoir. When a clip is used more than once in the Mixer, the clip occurrences can share the data from the Reservoir, saving memory.