Interpol provides multi-jurisdictional investigation. particularly across Alliance worlds, but also when an investigation strays beyond. In addition it provides support assets for individual Alliance forces when heavy backup is required.
Numerically, Interpol is smaller than most systems police forces, but is well trained, well equipped and well respected, Interpol also acts as a direct intervention force on behalf of the Assembly.
It is named after a long defunct organisation dating back more than a millennium from Earth that facilitated international police cooperation.