The is part of a switch structure. It defines a case value, that is, a value that is compared with the whole structure's switch value to determine whether the actions defined under this will be performed. The head of a switch structure is the . This is where the is defined.
A special case of the is the . It defines the actions that will be performed if none of the other case node conditions evaluates to true. Because the does not make any comparisons with the switch value, it has no value field. All that you have to specify is a description.