These are for use in pose mode or armature edit mode, to let you quickly traverse up and down a chain of bones.
It's quite useful for bones that are in hard-to-click places.
The tools are:
*Select parent/child ( [ and ] )
selects the parent or child of the active bone, deselecting the original active bone
* Extend select parent/child (shift [ and shift ] )
selects the parent or child of the active bone, adding to the selection
Thanks to Joshua for reviewing this so promptly!
PS. I'd like to use these [ and ] keys more widely in blender as consistent 'select next / previous' tools. I can imagine it being very useful for a lot of things like keyframes, nodes, mesh edges, etc.
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