xslt string units

Hello,

Does the string functions (string-length, substring, ... ) work with Unicode character units, or the implementation code point units ?
If they count as implementation code point units, does it have a fixed type (like utf16) ?  Or is it implementation dependant ?

In other words, does the Unicode characters above or equal 0x10000 count as 1 or 2 xslt string characters ?

Thanks for your replies

Vincent De Groote