Where a document cited in the European search report is particularly relevant, it is indicated by the letter "X" or "Y". Category "X" is applicable where a document is such that when taken alone, a claimed invention cannot be considered novel or cannot be considered to involve an inventive step.
Category "Y" is applicable where a document is such 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, such combination being obvious to a person skilled in the art. However, if a document (a so-called "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 indicated by the letter "X", i.e. not "Y", and the document referred to (the "secondary" document) is indicated as "X" or "L" as appropriate.