I agree with
@danikoski, I am pretty certain the Tomorrowland Terrace sit-down version is first come, first assigned. Years ago they had assigned tables with little name plates on them, but that's not the case any more. When I did this version back in April I arrived (purposely) late - 8:40ish if I'm not mistaken. My table was nearly all the way to the right of the terrace which I'd say is probably an inferior location (in exaggerates the off-centered viewing of this location).
It seemed like the CMs had filled tables from the left to the right (if standing looking at the castle from there). I'm not sure but the left side viewing might be a bit better, and so arriving at a more prompt time may be helpful? Not 100% sure about this though.
Hope that helps.