Map: the "volatile" parameter. By default, "map" creates cacheable variables [1]. With this parameter it creates a non-cacheable variable. An original idea was to deduce the cacheability of the "map" variable by checking the cacheability of variables specified in source and resulting values, but it turned to be too hard. For example, a cacheable variable can be overridden with the "set" directive or with the SSI "set" command. Also, keeping "map" variables cacheable by default is good for performance reasons. This required adding a new parameter. [1] Before db699978a33f (1.11.0), the cacheability of the "map" variable could vary depending on the cacheability of variables specified in resulting values (ticket #1090). This is believed to be a bug rather than a feature.