These commands are part of the LaTeX3 Project
xparsepackage. This question is to satisfy curiosity…
xparse documentation released on 25 Nov 2014:
\DeclareDocumentCommandwill always create the new definition, irrespective of any existing with the same name.
\NewDocumentCommandwill issue an error if has already been defined.
Is this the equivalent of
In what situation would I need this?