Macro definition. An Image Serving protocol fragment consisting of one or more complete commands, separated by '&' characters.
Macro definitions may invoke other macros, but macros must be defined before they can be referenced. Thus, a macro definition can invoke a macro which occurs earlier in the same macro definition file. Or, it may invoke a macro from the default catalog, assuming that the invoking macro definition is not in the same file. Macros can define and use custom variables.
Text string. Required.