The Presidential Records Act of 1978 states that all official records created by a President pass immediately to the custody of the Archivist of the United States on January 20th, at 12:00 o'clock (Inauguration Day), as the President's successor is sworn in. However, there are distinctions in the Act between Presidential and personal records. This distinction affects how NARA administers the records.
It is very complicated to determine what information is personal and what is Presidential. Sometimes family members or associates will donate personal papers to Presidential Libraries, but those types of records are voluntary, and may be offered with provisions for access outlined in a donor's deed of gift. Campaign materials and pre-transition materials are among the records that are considered personal and political and are not governed by the Presidential Records Act. However, if something is used in the President's conduct of business, it is considered a (Public Link) Presidential record. In recent years, the advent of electronic communication has further complicated the distinction between personal and Presidential records. But legal cases help to create a framework based on constitutional law for determining when personal records become Presidential.