There is 'Male' and 'Female', and below that should be 'Bones' or something similar.
Normally, it is completely invisible. But when in edit mode or if a debug option is turned on, it displays the bones as lines, joints as circles and all attachment points as smaller differently-colored circles.
The idea is to get rid of invisiprims, which are a work-around at best and buggy in situations where you want a full-prim avatar.
The Male and Female models both have sliders that are only available in those forms. Similarly, the Bones would have far less sliders (being mostly size and scale) and ignoring many of the finer chin/eye/etc changes that would be unnecessary.
Perhaps possible to also scale below and above the norms for a standard avatar when using a bones avatar?