From Valve Developer Community
It's used for Survival mode maps to specify where gas canister missiles should launch from.
Note:If this entity does not exist it will be create at the origin of trigger_survival_playarea.
Note:This entity can be placed in 3D Skybox and its location will be translated with other contents of 3D sky.
- Name (targetname)
- The name that other entities refer to this entity by.
- Entity Scripts (vscripts)
<scriptlist>(New with Left 4 Dead 2)
- Space delimited list of VScript files (without file extension) that are executed after all entities have spawned. The scripts are all executed in the same script scope, later ones overwriting any identical variables and functions.
- Script think function (thinkfunction)
<string>(New with Left 4 Dead 2)
- Name of a function in this entity's script which will be called automatically every 100 milliseconds (ten times a second) for the duration of the script. It can be used to create timers or to simulate autonomous behavior. The return value (if present) will set the time until the next call. Try to avoid expensive operations in this function, as it may cause performance problems.