The President often gets 10,000 letters a day. Virtually all must be opened, read and answered by the White House mail staff.
So that letters from friends get to the President and family unopened, all close friends are given a sequence of numbers to write on the outside of the envelope. The code changes with each President. Ronald Reagan’s code was described as a number with a special meaning to Reagan and his wife. Jimmy Carter used an old phone number of Rosalynn’s.
From: Big Secrets By: William Poundstone