in the 1950s, the format of bsp designations was changed to a nine-digit numerical format, written in three groups of three digits. the first three digits referred to the "division", which indicated a broad subject area, such as "subscriber sets" or the no. 1 ess. the next three digits indicated a specific subject area, such as specific type of equipment used within the major division subject. the final three digits indicate the serial number of the document. the content could be general descriptive information, information on wiring and connections, test procedures, or piece-part replacement and repair information.