Rules for Sharing Skeleton Assets
An important feature of Skeleton assets is that a single Skeleton asset can be used by multiple Skeletal Meshes.
Best of all, the bone hierarchies in each one do not need to be identical. They do, however, need to have the same general structure.
Most importantly, the bone hierarchy from one Skeletal Mesh to another cannot be violated
This means that the bone name and order of your bones must be consistent.
For example, consider a 3-bone limb with bones named 1, 2, and 3:
If you had a second Skeletal Mesh that you intended to use the same Skeleton asset, it would need to keep those bones in the same order with the same names.
You can, however, add bones that are peripheral to the hierarchy (characters that don't have those additional bones would just not use that data). So for example, your new hierarchy could look like this:
However, in order for both Skeletal Meshes to use the same Skeleton asset, you cannot re-order the hierarchy or re-name the bones. For instance, this is not valid:
In this instance, the second Skeletal Mesh uses a different bone hierarchy and would need a new Skeleton asset.