This small guide will tell you your HUD icons and what they do, these icons may popup ingame so look carefully.
Constructables
Allied Constructable
Axis Constructable
Neutral Constructable Hints
Backstab: The backstab icon indicates that you can stab a player in the back.
Breakable: This icon indicates you can break something with or knife or gun.
Constructable: This icon means an engineer can construct something there.
Disarm: This icon means you can disarm the dynamite you are facing.
Dynamite: Meaning this object can be destroyed with an engineers dynamite.
Activate: You can open doors, use and activate things when this icon appears.
Ladder: Meaning you can climb up that when the icon appears.
Revive: This icon will only show up if you are a medic, which means you can revive the teamate with this icon over his dead body.
Tank Destructible Only: Means the thing your crosshair is pointing at can only be destroyed by the cannon of a tank, not the engineers dynamite.
Uniform Steal: Meaning you are a covert ops and can steal the pants off a dead enemy and go disguised as the enemy.
Satchel Destroy When this icon comes up it means you are a cover op and the object can be destroyed with a satchel.
Team DoorsTeam Doors can only be used by players of the appropriate team or an enemy Covert Ops in disguise. The Team Door will remain open for a short time before it closes automatically. You may want to check behind you to make sure no enemy player sneaks in behind you. If you attempt to open an enemy Team Door and you're not a disguised Covert Ops, you'll see this icon.
Hope this comes in handy!
