Concatenates all the values of the input sequence into one string delimited by whatever string you choose to use as the delimiter. The string-join of an empty set is the empty string. Not available in XSLT1.
The example below contains four separate customer numbers 2 4 6 and 8. The constant character supplies a hash character "#" as the delimiter.
Result = 2#4#6#8
If you do not supply a delimiter, then the default is an empty string, i.e. no delimiter of any sort. Result = 2468.