@JosephWright, @PhelypeOleinik, @egreg, @HaraldHanche-Olsen, thank you, it took me some time to figure out.
% WORKING WITH PREDEFINED REGISTERS
\setbox0=\hbox{hello}
%\box0 % display once (empty after first display)
%\box0 % empty
\copy0 % not empty (can be displayed repeatedly)
\copy0 % not empty
% WORKING WITH USER-DEFINED REGISTERS
\newbox\myBox
\setbox\myBox\hbox{abc}
%\box\myBox % display once (empty after first display)
%\box\myBox % empty
\copy\myBox % not empty (can be displayed repeatedly)