I had a need to figure out what the orders.dat unknowns do, so might as well document that here:

• Unknown 4 - If set, overrides subunit's order as well when the main unit is ordered.
  Otherwise subunit's order doesn't change when issuing this order to the main unit.
  
• Unknown 7 - If set, and the unit is shift-queued a waypoint path, the unit will slow down at each waypoint. Otherwise the unit only slows down at the end. (The exact behaviour is to slow down if there are no additional waypoints when the current order is started to be executed.)
  
• Unknown 9 - If the unit is disabled with a spell (Lockdown/Maelstrom/Stasis), it is targeting a unit, and this flag is set, it will keep targeting once the spell wears off. Otherwise it will lose its target. If the unit is targeting ground, it will keep the target regardless of this flag's state.
  
• Unknown 10/Can be obstructed - If the unit is a ground unit, targeting ground will cause the target to be clipped to the closest walkable position. Interestingly this flag is set for maelstrom, which makes it annoying to hit flying units on top of unwalkable terrain. <.<
  
• Unknown 11 - If the unit is damaged by an attacker it cannot attack back, and this flag is set, it will interrupt the order to run away from the attacker. Ai-controlled units may disregard this flag in some situations.
  

Other unknowns are unused, bw only reads them in debug/beta builds for some assertions:

• Unknown 2 - The order is a secondary order.
  
• Unknown 3 - Non-subunits should be able to execute this order.
  
• Unknown 5 - Subunits should be able to execute this order.
  
• Unknown 12 - Order requires an unit that is able to move.