Classification titles may contain a phrase in brackets “( )” referring to another place in the classification. Such a phrase, called a reference, indicates the subject matter covered by the place (or places) referred to by the reference. A reference is applicable to the place where they appear and to all hierarchically lower places.

There are two types of references, limiting and non-limiting references.

Limiting References – Scope-limitation

Limiting references are included in the classification schemes and in the definition, when available.

Scope-limitation reference indicates the exclusion of a specified subject matter from the scope of the current classification place, even though it is apparently covered by the title of the place where the reference appears. The reference also specifies the place(s) where this subject matter is classified. This type of reference is very important for the proper understanding and use of the classification place.

Example:

A45F 3/00 is the place for classifying “Travelling or camp articles”, which include “travelling rugs”. However, the limiting reference states that “travelling rugs” are classified at A47G 9/06.

Limiting References – Precedence

Limiting references are included in the classification schemes and in the definition, when available.

This type of reference starts with a classification symbol(s) followed by the phrase “takes precedence”. A precedence reference is used when a subject matter is either classifiable in two places or when different aspects of the subject matter are covered by different places and it is desired that such subject matter should be classified in only one of those places.

i. A precedence reference to a place which is a subset of the place where the reference stands has the same function as a scope-limitation reference.

ii. A precedence reference to a place which is not a subset of the place where the reference stands serves as a classification rule for combination-type subject matter.

Example (hypothetical classification scheme):

10/00 Mechanical means (20/00, 30/00 take precedence)
20/00 Hydraulic means
30/00 Chemical means
Since hydraulic means are a type of mechanical means, it excludes a subset of the matter covered by 10/00 and places it in a different place. The precedence reference from 10/00 to 20/00 has the same function as a scope-limitation reference saying "(hydraulic means 20/00)".
The precedence reference from 10/00 to 30/00 does not exclude chemical means per se, since chemical means per se are not covered by 10/00. Its function is to exclude subject matter that would otherwise be classified in both 10/00 and 30/00, i.e. combinations of mechanical means and chemical means. It has the same function as a scope-limitation reference saying "(mechanical means in combination with chemical means 30/00)".

Non-limiting References – Application oriented

Non-limiting references are presented in the definition of the classification.

Application-oriented references usually appear in function-oriented places. These references direct users to places where the subject matter of the current classification place is covered if it is specially adapted, used for a particular purpose or incorporated in a larger system.

Application-oriented references are listed in tabular form under the heading “Application-oriented references” in the definitions.

Example: When considering the subject matter covered by subclass H01S, its definition displays the following application-oriented references:

Non-limiting References – Out of a residual place

This type of reference appears in residual place and contains a list of subject matter covered by the current residual place.

References out of residual places are listed in tabular form under the heading “References out of a residual place” in the definitions.

Example: When considering the subject matter of light sources, the main group F21K99/00 is residual to the whole of the IPC, and the following are examples of references to other places which provide for the subject matter under consideration:

Non-limiting References – Informative

Informative references indicate the location of subject matter that could be of interest for searching, but is not within the scope of the classification place where the reference occurs.

Informative reference is different from application-oriented reference as an informative reference usually points from an application-oriented place to a function-oriented place while an application-oriented reference usually points from a function-oriented place to an application-oriented place.

In the definitions, informative references are listed in tabular form under the heading “Informative references”.

Example: Informative references under the definition of F21K 5/00: