iGIS stores all its vector data in a SQLite database on the iOS device. A layers defined filter text is applied at the SQLite database level as the WHERE clause component of a SQL SELECT query.

i.e. a defined filter parameter of:

Unit_type IN (‘GP’,'SP’) AND source = ‘GCCT’

would be applied to the target layer as;

SELECT * FROM <target layer> WHERE Unit_type IN (‘GP’,'SP’) AND source = ‘GCCT’;

Filter text clauses just need to be SQLite SQL compliant. http://www.sqlite.org/