A reference designator unambiguously identifies the location of a component within an electrical schematic or on a printed circuit board.
The IEEE 315 standard contains a list of Class Designation Letters to use for electrical and electronic assemblies.
This standard, along with IEEE 315-1975, provide the electrical designer with guidance on how to properly reference and annotate everything from a single circuit board to a collection of complete enclosures.
ASME Y14.44-2008[1] and IEEE 315-1975[2] define how to reference and annotate components of electronic devices.
Subsequent demarcation are called assemblies and always have the Class Letter "A" as a prefix following by a sequential number starting with 1.