The neck is inserted into a pre-routed opening in the body (which is commonly called a "pocket"), and then joined using three to four screws.
As the pressure of screw heads damages the wood surfaces, and the undistributed stress could put the instrument body at structural risk, typically a rectangular metal plate (or a pair of smaller plates) is used to secure the joint and re-distribute the screw pressure more evenly.
The plate can then be used to emboss a manufacturer's logos, stamp serial numbers, or include decorative artwork.
[2] The term "bolt-on" is often a misnomer, introduced mostly by Fender whose electric guitars and basses have largely had component necks held to the instrument's body with wood screws.
Many of these views are highly subjective and relative; instrument manufacture varies widely and, lacking parallel objective factors, any particular claim might not be applicable generally.