Juergen Hannappel wrote:

A registered chises has a kind of flange at it's tang, just at the
point where the tang enters the handle. This flange (or bolster) is
there to bear the brunt of the axial force exertet on the tang-handle
joint when the chisel is heavily hammered on.

There is another definition of "registered chisel", where the sides are
square (ie. "registered") to the bottom. This is distinct from a
"bevelled chisel" where the sides are beveled. Typically this kind of
"registered chisel" is thinner than a full-on mortise chisel.

Then there's yet another theory
(http://thebestthings.com/newtools/sorby_framing.htm) that says "The
term "Registered" Chisel has its roots in the fact that up until modern
times, unique handle designs were often "registered" with the British
government for copyright protection.".

Chris