Conditional Compilation uses Macros to decide whether to include or exclude blocks of code from being executed; a typical usage of this is to enable/disable debug code.
${ifdef [!]macro-name} [data]
Conditionally executes code if macro-name is defined; if the exclamation mark is included before the macro-name then the code will only be executed if the macro-name is not defined. If data is given, then the ifdef command is an inline command, otherwise it becomes a grouped command and must be paired with a corresponding endif.