_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).

_thisScript

In expression executed by execVM or spawn, this variable stores the Script handle.

In FSM executed by execFSM, this variable stores FSM ID (the same as is returned by execFSM command).

This variable stores Event Handler ID (the same ID returned by command, which adds Event Handler) and is available for certain Event Handlers. See Arma_3:_Event_Handlers to find out which event handlers support it.

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" line): An Array containing all group members that completed the respective waypoint.
(thisList in a trigger's deactivation field seems to be undefined, and should not be used. It will either return an empty array, or possibly the contents of a different trigger.)

In Triggers ("Condition" or "On Activation/Deactivation" line): thisTrigger refers to the trigger itself.