1. At a banknote evaluation machine comprising a sensor unit with at least one imaging optical sensor for creating digital images of bank notes, a processor and a memory, a method for determining a class reference data record for the classification of value documents, in particular bank notes, the method comprising the following steps: the banknote evaluation machine creating a class reference image by means of a plurality of reference images of already classified value documents of same class, the plurality of reference images being created using the imaging optical sensor; and creating, by using the processor, a class reference data record having at least one class reference parameter with the aid of a user-defined region having at least one optical feature within the class reference image, the class reference parameter being coupled to at least one quantitatively determined property of the user-defined region, the class reference parameter being created by forming a ratio of pixel values from at least two different locations on a value document, the ratio being formed from a pixel value averaged over a specified region of an infrared-absorbing section of the value document to a pixel value averaged over a specified region of an infrared-transparent section of the value document; wherein the pixel values of a respective pixel of the class reference image are at least one of the average pixel values or median pixel values of the corresponding pixels of the plurality of reference images of already classified value documents of the same class, wherein each class reference parameter has assigned thereto a tolerance range, the tolerance range for each class reference parameter being variable and being dependent on the quantitatively determined property of the user-defined region; and wherein the class reference image of a class of value documents is represented, for analysis, on a display unit by means of a graphical user interface.