When an Armature object is in Pose Mode (CTRL+TAB on armature, or use mode selector menu), you can assign for each individual bone a specific drawing type.

This is useful for example to create a 'base bone', or to design driver handles to control facial expressions via Shape Keys.

To do this, just fill in the name of a Mesh object in the Button named "OB" in the "Armature Bones" Panel. Note that only Mesh object types are supported.

On the location of the bone, using the bone's transform, then the Mesh is drawn. In 'solid drawmode' it will show in a color as set in the Mesh-object's material.

Read this post in the Orange blog about character animation tools.

For each Armature you now can set 16 visibility layers. For each Bone you have 16 visibility layers as well, which together defines which of the Bones are visible.

The Armature layers you can find in the "Armature" Panel, the Bone layers in the "Armature Bones" Panel (see image of previous section).

When the object is in Pose Mode, the hotkey M will alllow to quickly set layers as well, a menu such as the image here will then pop up.

The Action editor will only show channels for visible bones.

Layers are only used to control visibility, to hide helper bones, or to group bones in a way that only the active 'handlers' are visible for editing. For the result of evaluating an Pose the layers don't matter.