In patent law, a search report is a report established by a patent office, which mentions documents which may be taken into consideration in deciding whether the invention to which a patent application relates is patentable.
[1] The documents mentioned in the search report usually form part of the prior art.
Letters are often included in search reports established for patent applications to indicate the relevance of the documents identified by the examiner.
For instance, the European Patent Office (EPO) uses the following letters in search reports or in the European Patent Register:[2][3] Combinations of different categories are possible.
[3] For instance, a document classified as "D, A", i.e. in categories "D" and "A", would be a document cited in the application and regarded as representing the technological background of the invention.