You can find it in the QlikView Desktop Help. I have pasted in the entry below.
subfield(s, 'delimiter' [ , index ] )
In its three-parameter version, this script function returns a given substring from a larger string s with delimiter 'delimiter'. index is an optional integer denoting which of the substrings should be returned. If index is omitted when subfield is used in a field expression in a load statement, the subfield function will cause the load statement to automatically generate one full record of input data for each substring that can be found in s.
In its two-parameter version, the subfield function generates one record for each substring that can be taken from a larger string s with the delimiter 'delimiter'. If several subfield functions are used in the same load statement, the Cartesian product of all combinations will be generated.
Examples:
(For three parameters)
subfield(S, ';' ,2) returns 'cde' if S is 'abc;cde;efg'
subfield(S, ';' ,1) returns NULL if S is an empty string
subfield(S, ';' ,1) returns an empty string if S is ';'
