Apply
Reference
Mode
Pose Mode
Menu
Pose ‣ Apply
Hotkey
Ctrl-A
Pose as Rest Pose
Conversely, you may define the current pose as the new rest pose (i.e. “apply” current transformations to the Edit Mode), When you do so, the skinned objects/geometry is also reset to its default, undeformed state, which generally means you will have to skin it again.
Pose Selected as Rest Pose
Same as Pose as Rest Pose but only applies to selected bones.
Visual Transform to Pose
TODO.
Assign Custom Property Values as Default
TODO.