It is used to measure ads and articles. The desk where articles are edited, headlines and captions are written, newspaper style is enforced and deadlines are either made or missed. Usually, these letters are put just after the copy they refer to. Some newspapers strictly enforce a rule that the dateline must say where the reporter was when the story was gathered. A foreign story gathered by phone at home, then, might run with no dateline.
Every paper has dozens in a day for the hundreds of parts that go into it.
You might ask what the deadline is for the piece you're working on, the deadline for the last type to be set or the time when the presses should start. An ad or editorial project that covers two facing pages. This name comes from the days when the heavy forms for newspaper pages, largely filled with lead type, were rolled around the composing room floor on heavy carts called trucks. Two pages for one project meant a double truck. Some newspapers put weather or lottery information in them.