Many attributes inherit their values from the enclosing property or the global scope. In certain circumstances, this is not desirable, e. g. for list attributes. A list attribute is any attribute that takes a comma separated list of values as argument.
allocate and
flags.task are good examples of commonly used list attributes. By defining values for such a list attribute in a nested property, the new values will be appended to the list that was inherited from the enclosing property. The purge attribute resets any attribute to its default value. A subsequent definition for the attribute within the property will then add their values to an empty list. The value of the enclosing property is not affected by purge.