There are 7 types of attribute data types:
An Unsigned type is listed but not used or currently described - it should be regarded as experimental. Experimental attribute types appear from time to time and are usually adopted formally or removed; the release notes normally give detail on these types.
Note that Action, Color, File, List, Set and URL are effectively all special forms of String attributes and share a common default - an empty string. Booleans default to false (which is not a string), Dates to 'never' and Numbers to 0 (zero).
