This is from the Expat Function Reference by Clark Cooper, which is a reference to the C api.
"XML_Parser XML_ParserCreateNS(const XML_Char*encoding, XML_Char sep)
Constructs a new parser that has namespace processing in effect. Namespace expanded element names and attribute names are returned as a concatenation of the namespace URI, sep, and the local part of the name. This means that you should pick a character for sep that can't be part of a legal URI."
So thats what this function is for. Now you know.