LaTeX template for use cases

You should try and keep you author's interface as simple as possible. I would create an environment to simplify this and add complexity and presentational aspects on the setup.

\newenvironment{usecase}
  \addheading{}{}
  \addrow{}{}
  \addmulrow{}{}
\end{usecase}

Here is a minimal example to implement this:

\documentclass{article}
\usepackage{booktabs}

\newcommand\addrow[2]{#1 &#2\\ }

\newcommand\addheading[2]{#1 &#2\\ \hline}
\newcommand\tabularhead{\begin{tabular}{lp{8cm}}
\hline
}

\newcommand\addmulrow[2]{ \begin{minipage}[t][][t]{2.5cm}#1\end{minipage}% 
   &\begin{minipage}[t][][t]{8cm}
    \begin{enumerate} #2   \end{enumerate}
    \end{minipage}\\ }

\newenvironment{usecase}{\tabularhead}
{\hline\end{tabular}}
\begin{document}
\begin{usecase}
  \addheading{Actor}{System user} 
  \addrow{Precondition}{The system, shows, in the form part of an object type, the number   indication.}
  \addrow{Postcondition}{A disconnected number indicating the type of `other constructed object'.}
  \addmulrow{Main path (M)}{\item User selects \ldots
                                   \item System demands \ldots}
\end{usecase}

\end{document}

You can add counters to automatically increment numbering, if you want. I couldn't understand your numbering scheme and hence left that out of the example.

To summarize, keep the author's commands simple (they only need to deal with the semantics) and place your presentational aspects in separate commands.

Had change Yiannis example a little bit. Add counter, table capture, change style of table to be relevant to ESKD standard, and remove rudiment definitions.

Now definition is:

\begin{usecase}{System user}
    \addrow{Precondition}{The system, shows, in the form part of an object type, the number indication.}

    \addrow{Postcondition}{A disconnected number indicating the type of `other constructed object'.}

    \addmulrow{Main path (M)}{
        \item User selects \ldots
        \item System demands \ldots}
\end{usecase}

And it's looks like:

And the source is:

\newcommand\tabularhead[1]{
\begin{table}[h]
  \caption{Action <<#1>>}
  \begin{tabular}{|p{0.4\linewidth}|p{0.55\linewidth}|}
    \hline
    \textbf{Action} & \textbf{#1} \\
    \hline}

  \newcommand\addrow[2]{#1 &#2\\ \hline}

  \newcommand\addmulrow[2]{ \begin{minipage}[t][][t]{2.5cm}#1\end{minipage}% 
     &\begin{minipage}[t][][t]{8cm}
      \begin{enumerate} #2   \end{enumerate}
      \end{minipage}\\ }

  \newenvironment{usecase}{\tabularhead}
{\hline\end{tabular}\end{table}}