Item types are the first component of a sequence type, together with the cardinality indicator. Thus, an item type matches (or not) a single item. For example, "foo" matches the item type xs:string.
There are three categories of item types:
Atomic types (W3C-conformant, additional js:null and js:atomic)
Structured types (JSONiq-specific)
Function types (W3C-conformant)
JSONiq uses a JSONiq-specific, implementation-defined default type namespace that acts as a proxy namespace to all types (xs: or js:). As a consequence, buitin atomic types do not need to be prefixed in the JSONiq syntax (integer instead of xs:integer, null instead of js:null).
All items match the item type js:item, which is a JSONiq-specific synonym for the W3C-confirmant item().
Figure 5. ItemType