This word, an abbreviation of in re, is the ablative case of the Latin word res. It means in the matter of. It is used by lawyers for the title of lawsuits, such as "in re John Doe deceased". It has passed into commercialese as an equivalent of the English preposition about. It has no business there, or in officialese. It is not needed either in a heading ("re your application for a permit "), which can stand without its support, or in the body of a letter, where an honest about will serve your purpose better.