They are also used to identify Resistance-friendly Combine weapons such as Hopper Mines and Sentry Turrets, seen by the spray-paint on them after being reprogrammed. Resistance fighters also wear the symbol on their armbands.
The rebels would spray-paint it near supply caches, and resistance bases and safehouses such as White Forest, Black Mesa East, and Kleiner's Lab. Gordon's Lambda logo on his suit and the work of the Lambda Complex became something of a legendary trademark symbol for the Resistance against the Combine, referring to the acts of Gordon Freeman during the Black Mesa Incident. The logo when used to signify a Resistance area or a Resistance weapons cache. A section of Black Mesa dedicated to teleportation research was also called the 'Lambda Complex,' and is identifiable by the Lambda logo at its entrance.
In the original Half-Life, Gordon Freeman's trademark HEV Suit was marked with a Lambda logo on the chest, as were other HEV Suits.
