In general the Bind Pose can be any user defined pose. However, Blender (currently) only supports binding to the Rest Pose.
However, since a few years Blender binds the mesh to the armature by using an Armature Modifier. The menu entry for parenting to armature deform has been kept mostly for convenience and ease of use.
A Rig is an animation control system for a Model. Rigs can use different methods for animating. For example it is possible to animate a mesh only by using empties and constraints, so no single bone is involved, yet it is an animation system.
The most common form of Rigs is based on Bones. This special form of a Rig is also named Armature.
Rigging is the process of creating the Rig.
However since in Blender we clearly have to distinguish the processes and workflows, we should also use precise terminology. Thus we distinguish between Rigging (make the rig) and Skinning (weight the mesh).
Skinning is the process of creating an association between a Mesh (the Skin) and a Rig. More specifically in the Skinning process we define associations for each bone of the Rig’s Skeleton to each vertex of a mesh. In practice, for a polygonal mesh character, each bone is associated only with a subset of the character’s vertices.
Portions of the character’s skin can be influenced by 2 or more bones, where each vertex of the affected area gets an influence factor to each associated Bone. This factor is also called Vertex Weight. Thus the influence of each bone on a specific Vertex is defined by its Vertex Weight.
The process of Skinning includes:
- Binding the Character to the rig in a specific Pose (basically establishing an association between the Rig and the Mesh)
- Weighting the Mesh to the Armature bones (create the Vertex Weights)
Weighting is the process of associating Bones to Vertices and giving each bone-vertex association a specific Vertex Weight, thus creating Bone Weight Maps for the mesh. In Blender the Bone Weight Maps are stored in Vertex Groups.