Now integrated in the LaTeX kernel as {ltcmd}. Formerly, {xparse} was a package from the LaTeX3 project that provided a powerful mechanism for defining new commands with various types of arguments.

Now integrated in the LaTeX kernel as ltcmd. Formerly, xparse was high-level package from the LaTeX3 project that provided a powerful mechanism for defining new commands with various types of arguments. The current xparse package gives access to deprecated command types and its usage is not recommended.

The main interface implemented in ltcmd is the \NewDocumentCommand macro and its variants, which improve on \newcommand and its variants, allowing commands with multiple optional arguments of various formats, verbatim arguments, and so on.