The thing you must always remember with ISO is it is closely related with Aperture(A) and Shutter Speed(SS). Generally it can be summarise into 4 cases but of course the combination can be thousands.
For slowest possible SS = narrow A + lowest base ISO - eg, use in night scene, bulb mode or landscape shoots, blur waterfall, etc on tripod
Slow to Mid SS = wide A + low ISO - eg, use in outdoor/indoor portrait with shallow DOF, static subject, etc
Mid to high SS = small A + high ISO - eg use in outdoor landscape with subject shooting, panning etc
whereas for fastest SS = widest A + highest ISO - used in sports or indoor/ dim, fast scene, freeze the aeroplane, etc
I always base my shoots on this basic principles