From Existing Models & Shapes

From Existing makes a chain from what is already in the scene: a model's insides, or shapes you blocked out.

Control	What it does
Skeleton from Mesh	Finds a chain inside the Loaded Mesh (else the Generated one): the largest balls that fit inside it, linked along its limbs. A clean start for a Quad Skin over a scan or a sculpt.
Chain from Shapes	Makes the selected Spheres, Capsules and Round Cones one chain: a ball per sphere and capsule end, joints they share merged, touching pieces linked. The shapes are replaced; Undo brings them back.
Grid	Skeleton from Mesh: how finely the model is sampled along its longest side. Finer finds thinner limbs and takes longer.
At most	Skeleton from Mesh: the most balls it makes.
Every	Skeleton from Mesh: how far apart the balls are along a limb, in their radii.
Smallest	Skeleton from Mesh: the smallest ball, as a share of the largest. Lower reaches further into fingers and tails.

Skeleton from Mesh can find several separate pieces in a model whose parts do not touch; the status line says how many balls and pieces it found. Use Link Balls or Join Chains to connect them.