The package keeps track of whether a command defined in a document preamble is actually used somewhere in the document. After the package is loaded in the preamble of a document, all \newcommand (and similar command definitions) between that point and the beginning of the document will be marked for logging. At the end of the document a report of command usage will be printed in the TeX log, for example: “mdash was used on line 25”; “ndash was never used”.