\makeatletter & \makeatother ¶Synopsis:
\makeatletter ... definition of commands with @ in their name .. \makeatother
Use this pair when you redefine LaTeX commands that are named with an
at-sign character ‘@’. The \makeatletter
declaration makes the at-sign character have the category code of a
letter, code 11. The \makeatother declaration sets the
category code of the at-sign to code 12, its default value.
As TeX reads characters, it assigns each one a category code, or
catcode. For instance, it assigns the backslash
character ‘\’ the catcode 0. Command names
consist of a category 0 character, ordinarily backslash, followed
by letters, category 11 characters (except that a command name can
also consist of a category 0 character followed by a single
non-letter symbol).
LaTeX’s source code has the convention that some commands use
@ in their name. These commands are mainly intended for package
or class writers. The convention prevents authors who are just using a
package or class from accidentally replacing such a command with one of
their own, because by default the at-sign has catcode 12.
Use the pair \makeatletter and \makeatother inside a
.tex file, typically in the preamble, when you are defining or
redefining commands named with @, by having them surround your
definition. Don’t use these inside .sty or .cls files
since the \usepackage and \documentclass commands already
arrange that the at-sign has the character code of a letter,
catcode 11.
For a comprehensive list of macros with an at-sign in their names see https://ctan.org/pkg/macros2e.
In this example the class file has a command
\thesis@universityname that the user wants to change. These
three lines should go in the preamble, before the
\begin{document}.
\makeatletter
\renewcommand{\thesis@universityname}{Saint Michael's College}
\makeatother