The function mapping definition sessionize_fm specifies a scalar subquery for the TIMEOUT column value:
SELECT timeout FROM ssqtbl WHERE username='John'
If TIMEOUT is not specified during function processing, the column value is resolved by processing the scalar subquery.
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='John'), CLICKLAG, EMITNULL;