This feature allows the usage of source mesh parts that have different but compatible skeletons.
This is an early-access feature that needs to be enabled manually. It can be done by going to your Customizable Objects properties panel and setting the "Enable Compatible Skeletons" checkbox property to true.
The first time you compile the CO after this, the reference skeletal mesh will be duplicated, and all the new bones it gathers will be added to this new reference mesh. As new skeletal mesh assets with new compatible skeletons are added to the CO and its children, new bones will be added to the reference skeletal mesh.
An alternative way of working with this new feature is to manually construct a new reference skeletal mesh with all the bones that will be potentially used in the skeletal mesh assets and set it as the reference mesh.