Difficulty
Sandbox, Realistic or Nightmare: how hard the world pushes back.
Sandbox is relaxed. You get a head start, money comes in easily and rivals are slow to pick on you. It is the perfect place to learn.
Realistic is a fair fight. Rivals play to win and your people expect to hear from you. Stay alert, keep busy, and victory is yours.
Nightmare is for veterans. Rivals are richer and sharper and gang up on you, rumours fill every silence, and there is no tutorial and no tips.