According to IUB classification of enzyme, 4th digit in enzyme refers to ?

The classification of enzyme was described in 1961 by enzyme commission of the international Union of Biochemistry (IUB). According to this classification, each enzyme code number or 'EC' number, consisting of four digits:- The first digit indicates the main class; that characterizes the type of reaction they catalyzed. Second digit indicates the subclass. This indicates the type of group involved in the reaction. Third digit indicates the sub-subclass, which denotes the substrate on which enzyme acts. The fouh digit specifies the individual enzyme. It indicates the systematic serial number and name. For example, lactate dehydrogenase (LDH) : EC (1.1.1.27) The first number indicates that the enzyme is an oxidoreductase. Second number indicates that the enzyme acts on the CH-OH group which is the donor of hydrogen. The third number indicates that the acceptor is NAD.