Max. power:
The most commonly communicated measurement, it indicates the spool’s maximum drag power.
Usable power:
The closest measurement to use, it takes into account the deformation of the reel combo (rotor, pick-up support, etc.) and indicates the drag casting weight: available for fighting against the fish.
Explicative video: https://www.youtube.com/watch?v=zqDGK0UdZnM