A signature mark is a letter, number or combination of either or both, which is printed at the bottom of the first page, or leaf, of a signature or section. This practise is to ensure that the bookbinder can order the pages and sections in the correct order.
A number of symbols used as binding signature marks were encoded in ISO 5426-2[1] and from there (to enable migration of data from the old standard) found their way into Unicode.
U+2619 (☙), REVERSED ROTATED FLORAL HEART BULLET, was added later. These latter two are the only codepoints in Unicode 4.0 to bear the annotation "binding signature mark". It is unlikely that Unicode will encode any more marks since they constitute metatextual and not textual information even though other symbols were used as binding signature marks with printers making something of a house style of the particular blocks of type they chose.