It encodes decimal numbers only – other characters are not allowed. The basic barcode consists of two blocks of 5 numbers, a start number and a check number at the end. These numbers may be printed below the bars if Optical Character Recognition may be required. The barcode is topped and tailed by a pair of 'guard bar' elements plus a guard bar element separating the two blocks. Each element is coded in two bars with two spaces, i.e. 30 bars in all.
Publishers, in particular, often also include an additional 2-digit (EAN 2) or 5-digit (EAN 5) supplementary code. In these cases, tick the appropriate option.