We are in the process of updating the user guide. The English user guide is up to date, the Chinese one will follow as well, here in the info in the English one:
A bone can be added to one or more skins so the bone is active only when a skin it is in is active. This allows a skeleton to have bones that are only used for certain skins. Bones that are not active do not affect runtime performance.
我们正在更新用户指南。 英文用户指南是最新的，中文用户指南也是最新的，这里是英文版的信息：
一个骨骼可以添加到一个或多个皮肤中，因此只有当它所在的皮肤处于活动状态时，骨骼才会处于活动状态。 这允许骨架具有仅用于某些皮肤的骨骼。 不活动的骨骼不会影响运行时性能。
