I got confirmation on the official forum that megaactions are activated when you first attain r00t rank.
suggested text:
While at the fields you may run field actions, as you run field actions you can get key events, reward events or be attacked by a virus. Exactly what events/virii are available depend on the keys, the field difficulty and a large amount of randomness. See tables below.
Field actions cost 10 stamina (-2,3 village:Science, -1 field:r00t powers, -2 field:overture, -1 ally:stalkergirl3)
Megaactions become available when you attain R00t rank for the first time, they cost X times the stamina and give Y times the reward.
Please insert links as appropriate. Possibly rename field actions to server actions, I am unsure as to the correct nomenclature.