The sample codes:
\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\noindent
\begin{NiceTabular*}{\textwidth}{XX}[hvlines]
X & X \\
\end{NiceTabular*}
\end{document}
I replace {XX} by {CC}, and the output:
The sample codes:
\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\noindent
\begin{NiceTabular*}{\textwidth}{XX}[hvlines]
X & X \\
\end{NiceTabular*}
\end{document}
I replace {XX} by {CC}, and the output:
{NiceTabular*} is not equivalent to {tabularx} (of the eponymous package) but to the standard {tabular*} (certainly less efficient). So, there is no X column in {NiceTabular*}.
\documentclass{article}
\setlength{\parindent}{0pt}
\usepackage{nicematrix}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{NiceTabular}{\linewidth}{@{\extracolsep{\fill}}ccccc}
one & two & three & four & five \
six & seven & eight & nine & ten \
eleven & twelve & thirteen & fourteen & fifteen
\end{NiceTabular}
\end{document}
The latest version of nicematrix (6.0 of 2021-08-10) provides an environment {NiceTabularX} similar to the classical environment {tabularx} of the package tabularx.
\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\noindent
\begin{NiceTabularX}{\textwidth}{XX}[hvlines]
X & X \\
\end{NiceTabularX}
\end{document}
nicematrixversion is 5.19. – Changlin Ai Jul 29 '21 at 08:20