Cfrgtkky

I am creating a new section called tsubsection taking over code practically 1:1 from subsection with the intention to introduce some modifications.

In this example

documentclass{report}

makeatletter

newcounter{tsubsection}

%renewcommand{thetsubsection}{@Romanc@tsubsection}                                                                                                                                                      

newcommandtsubsection{@startsection{tsubsection}{2}{z@}%                                                                                                                                                

                                     {-3.25ex@plus -1ex @minus -.2ex}%                                                                                                                                    

                                     {1.5ex @plus .2ex}%                                                                                                                                                   

                                     {normalfontlargebfseries}}

%newcommand*l@tsubsection{@dottedtocline{2}{3.8em}{3.2em}}                                                                                                                                               

makeatother

begin{document}

tsubsection{Methods}

Some text

end{document}

the output is

Where does the extra "Methods" in the head of tsubsection come from?

The extra Methods shows up because you didn't define tsubsectionmark, then the command:

csname tsubsectionmarkendcsname{Methods}

(which shows up somewhere deep in the definition of @startsection) becomes:

relax{Methods}

which writes Methods to the PDF.

You have to define the tsubsectionmark to add some page marking (with markright or markboth) or to simply consume the argument:

newcommandtsubsectionmark[1]{}

Working code:

documentclass{report}

usepackage{trace}

makeatletter

newcounter{tsubsection}

%renewcommand{thetsubsection}{@Romanc@tsubsection}

newcommandtsubsection{@startsection{tsubsection}{2}{z@}%

                                     {-3.25ex@plus -1ex @minus -.2ex}%

                                     {1.5ex @plus .2ex}%

                                     {normalfontlargebfseries}}

newcommandtsubsectionmark[1]{}

%newcommand*l@tsubsection{@dottedtocline{2}{3.8em}{3.2em}}

makeatother

begin{document}

tsubsection{Methods}

Some text

end{document}