References are statements enclosed within parentheses in titles. The references point to other classification places that may be of interest. References apply to the classification place they appear and to all hierarchically lower classification places, unless stated otherwise.
Examples of References:
B64C AEROPLANES; HELICOPTERS (air-cushion vehicles B60V)
(A) Limiting references
A limiting reference is found in the group titles of the scheme and exclude specific subject matter from the scope of the classification place, when this subject matter would otherwise fulfill all the requirements of the classification place (or would be covered by that place). Limiting references are pertinent for classification purposes. Limiting references limit the scope of a place, thereby avoiding overlap.
A01F 7/02 . With rotating tools (threshing cylinders or concaves A01F 12/18)
(B) Precedence references
A precedence reference is a special example of a limiting reference that always refers to another group or groups taking “precedence” within the same subclass. The purpose of a precedence reference is to remove overlap between two similar groups.
G02B 1/00 Optic elements
1/04 . made of organic materials (1/08 takes precedence)