Use the sorting tag for questions involving arranging things in a defined order: sorting lines alphabetically, bibtex entries by date or author, emails by sender, completion candidates by various criteria, etc.
Emacs provides basic text-sorting functions in library sort.el. This includes
sort-linessort-paragraphssort-pagessort-fieldssort-numeric-fieldssort-columnsreverse-region
Details are provided in the manual node (emacs) Sorting. Further details are provided in the Elisp manual, and many packages provide sorting functions for their particular subject.
