A signature mark is a letter, number or combination of either or both, which is printed at the bottom of the first page of a leaf. This practise is to ensure that the bookbinder can order the leaves in the correct order.
A signature is a printed sheet that is folded for binding as 4, 8, 16, 32, 64, or 128 pages (and also occasionally as 24 or 48).
A number of symbols used as binding signature marks were encoded in ISO 5426-2: and from there (to enable migration of data from the old standard) found their way into Unicode. 0x32 REFERENCE MARK, 0x34 MALTESE CROSS and 0x36 RIGHTWARDS LEAF ARROW were unified with U+203B (※), U+2720 (✠) and U+2767 (❧) ROTATED FLORAL HEART BULLET (= hedera, ivy leaf) respectively, but 0x37 LATIN CAPITAL LETTER SIDEWAYS Q had to be added into Unicode as U+213A (℺), ROTATED CAPITAL Q. 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". Although other symbols were used as binding signature marks, printers making something of a house style of the particular blocks of type they chose, it is unlikely that Unicode will encode any more marks, since they constitute metatextual and not textual information.