Skel Editor
The Skel Editor edits .nusktb files like model.nusktb. The model.nusktb contains the model's bones. SSBH Editor allows editing additional data associated with each bone for editing custom skeletons. Editing the model.nusktb is usually and unnecessary for many mods and is usually not recommended since it can lead to animation issues in game and cause desyncs online.
The Skel Editor has two main display modes. The list display mode shows the bone entries in the order they appear in the .nusktb file. This mode supports editing the bone's parent and billboard type. Reorder bones by dragging and dropping the handles to the new location in the list.
The hierarchy display mode shows the bones in the skeleton as a tree. This is the default way to display bones in a skeleton in applications such as Blender or Maya. Each bone appears under its parent bone. This mode currently doesn't support editing features.
The Skeleton menu contains features for dealing with bone ordering. Clicking Skeleton > Match Reference Bone Order will prompt the selection of a reference model.nusktb file. The reference .nusktb file is used to reorder the bones such that additional bones appear at the end. What properties need to be enforced for bone order is still an area of research. The latest version of the Smash Ultimate Blender addon handles this automatically when you select a .nusktb file before exporting.