If you do not specify the GROUP keyword on a CLASSDEF statement, no groups are explicitly assigned to the transport class. A group that you do not assign to a transport class is called an undesignated group. The set of all undesignated groups is collectively referred to on the CLASSDEF keyword and system commands by the pseudo-group name UNDESIG. The undesignated groups are implicitly assigned to the transport classes whose CLASSDEF statement did not specify the GROUP keyword. The undesignated groups can be explicitly assigned to a transport class by specifying GROUP(UNDESIG) on the CLASSDEF statement. After IPL, additional groups (including the UNDESIG set of groups) can be explicitly assigned to a transport class using the SETXCF START,CLASSDEF command or the SETXCF MODIFY,CLASSDEF command.