That means each of the three phases is rated for 104A. Some ESC makers would incorrectly take that value and multiple ny 3 to get 312A for the ESC rating. But it's anyone's guess if that's cont, burst, or whatever.
I'd take that to be 104amps total continuous rating at room temp (25*C). When things warm up, that rating drops- 'de-rating curve' I believe its called, so a hot esc will be less tolerant to bursts in current vs a nice cool one...
And if that is true, then you can consider that 104A per phase value more like ~50A per phase because FET junction temps are usually quite a bit higher than FET case temps, and FET case temps are higher than heatsink temps. Unless of course, you run your ESC submerged in liquid nitrogen.
And Neil, you are right, it's called temperature derating.