9.2.1 Particularly relevant documents 

Where a document cited in the search report is particularly relevant, it is labelled with the letter "X" or "Y". Category "X" applies where a document shows on its own that a claimed invention cannot be considered novel or to involve an inventive step.

Category "Y" applies where a document suggests that a claimed invention cannot be considered to involve an inventive step when the document is combined with one or more other documents of the same category and this combination is obvious to a person skilled in the art. However, if a document (called a "primary document") explicitly refers to another document as providing more detailed information on certain features (see G‑IV, 8) and the combination of these documents is considered particularly relevant, the primary document is labelled with the letter "X", i.e. not "Y", and the document it refers to (the "secondary document") is labelled with "X" or "L" as appropriate.