Characteristics of Writing That Makes Something Happen
Public writing has a very clear purpose (to promote a local cause or event; to inform or explain an issue or problem; to persuade others to act; sometimes even to entertain).
It is intended for a specific audience and addresses those people directly.
It uses the genre most suited to its purpose and audience (a poster to alert people to an upcoming fund drive, a newsletter to inform members of a group, a brochure to describe the activities of a group, a letter to the editor to argue for a candidate or an issue), and it appears in a medium (print, online, or both) where the intended audience will see it.
It generally uses straightforward, everyday language.