Randomly, by battle type. The automation, as stated, will list it in the forcedec that is issued when the 18-hour window closes, or for multi-drop battles, the multi-drop tracker will generate them randomly.

When I say "randomly", this does not mean even distribution; the distribution is weighted by automation configuration "knobs". For instance, currently the standard (mech, ind theft) raid is about 22% chance of 350 tons, and 33% chance of any of 400, 450 and 500 tons.