Once an \hbox has been declared that can grow arbitrarily wide, the mode that is invoked is the Restricted Horizontal Mode.

One example of being within a restricted horizontal mode occurs when you use \multirow command from within a table. No use of any symbols or escape sequences or even attempt to type parentheses or other special characters. for example if one was to try to type (?>blah) into this using \verb#(?>blah)# there's an error or \begin{verbatim}(?>blah)\end{verbatim} which makes this difficult to insert symbols into that first column of a multirow. I have also searched many text references to find the answer to this, and have come up empty handed.