The function mapping definition sessionize_fm specifies a TIMEOUT parameter derived from a scalar subquery that includes the system variable USER.
If TIMEOUT is not specified during function processing, the column value is resolved by processing the scalar subquery:
SELECT timeout FROM ssqtbl WHERE username=USER
CREATE FUNCTION MAPPING sessionize_fm FOR sessionize SERVER coprocessor USING InputTable IN TABLE, ModelTable IN TABLE, ClusterTable OUT TABLE, TIMECOLUMN, TIMEOUT(SELECT timeout FROM ssqtbl WHERE username=USER), CLICKLAG, EMITNULL;