Selection in Pose Mode is very similar to the one in Edit Mode, with a few specificities:
You can only select whole bones in Pose Mode, not roots/tips…
You can select bones based on their group and/or layer, through the Select Grouped pop-up menu Shift-G:
- To select all bones belonging to the same layer(s) as the selected ones, use the In Same Layer entry Shift-G 1.
- To select all bones belonging to the same group(s) as the selected ones, use the In Same Group entry Shift-G 2.
- Keying Set