These covers are those which are prepared and got postmarked by philatelist on particular event with or without cachet.  Themes of events have no limitations. To name a few.... they may be of achievements of individual or nation, paying tribute to Death of some personality, documenting some historic event of national or international importance or commemorating anniversary or centenary of some celebrity or event. These covers are creative masterpieces of philatelists' imagination and efforts because in many cases, philatelists have tried to commercially use these covers. Such covers make postal history and create niche in hearts of philatelists. They  give a new postal dimension to the event and in turn become more desirable to philatelists.