_this

In Scripts/Functions: The given arguments that were passed by a call, exec, execVM or spawn.
For the arguments in scripts executed by mission-made actions or Event Handlers, check addAction and Event Handlers.
In Addons' config files: An array with the current object as its first (and only) element.
In Addons' custom UserActions: Unit that is activating the action.
In Dialogs: An array with the current control as its first (and only) element (only available during runtime).

this

In Addons' config files (e.g. UserActions, onInit, etc.): The object the entry belongs to.
In Dialogs: The player unit that activated control (e.g. via mouseEnter). false if non-activated.
In Object's "Init" line: The object the init line belongs to.
In Triggers
- "Condition" line: The Boolean return value of the trigger's conditions (e.g. "East Not Present" etc).
- "On Activation/Deactivation" line: Seems to always return false.
In Waypoints
- "Condition" or "On Activation/Deactivation" line: The group leader that completed the respective waypoint (or driver, if waypoint is assigned to a vehicle).

In Triggers ("Condition" or "On Activation/Deactivation" line): An Array of objects that are currently fulfilling the trigger's condition (same as the what is returned by the list command).
In Waypoints ("Condition" or "On Activation/Deactivation" line): An Array containing all group members that completed the respective waypoint.