Predicate is consisted of a C++ Qt-only library and set of plugins. Because there is no dependency on Kexi, Calligra or even on KDE libraries, Predicate is available for very wide audience. Kexi 2.5 will use it too, thus deprecating KexiDB.
Predicate is widely free/libre software available under the GNU Lesser General Public License (LGPL). Thus, developers are free to use Predicate in applications and extend it for specialized use cases.