Suction fittings are plumbed to pumps, which suck in water and push it out though jets. They have anti-vortex grilles, to prevent entrapment, and typically quite large and deep, for safety reasons. Suction fittings are typically mounted on vertical surfaces in the footwell of the spa.
Floor drains are designed strictly to drain the spa by gravity. Floor drains are not designed to be connected to a pump.