Logical entity
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Logical entity is an entity whose position in the map doesn't affect its functionality in any way. They affect the game, creating or adding to various environmental and game controlling systems, such as AI managers, math counters, controlling fog, dynamic shadows, GUI elements, choreography and more.
In FGD they are defined using either @PointClass (or @FilterClass in case of filters) and thus in Hammer placed by Hammer Entity Tool.
If they don't need to directly or at all relay information to clients they are usually a subclass of CLogicalEntity thus not wasting an edict (some exceptions: logic_auto, logic_script, logic_choreographed_scene) otherwise they are usually direct subclasses of CBaseEntity, CPointEntity or CRulePointEntity.
Examples
Map properties
Entities that can be thought of as additional properties of the map and only one should exist at a given time.
Event detectors
Fires outputs based on some conditions occurring in-game like round starting, map loading for the first time or in certain gamemode.
Game controlling systems
Note:Many are spawned automatically by the game