There are 12 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, Font, List, Set and URL data types 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).