Use

All entities have a Use input, though many do nothing in response to it. There are four ways to trigger it:

1. A player looking at the entity and invoking the +use command (typically by pressing their e key)
2. An entity firing it via an output
3. An entity firing an output that does not specify an input on the target entity
4. An entity directly calling the C++ function

Use typically performs the action players most expect when interacting with the entity (e.g. opening/closing a door, picking up a physics object).

Programming

Although there is always a Use input, the actual Use function of an entity starts null. It can be assigned at any time with SetUse( void *SomeFunc(CBaseEntity *pActivator, CBaseEntity *pCaller, USE_TYPE useType, float value) ).

It doesn't appear possible to specify USE_TYPE or value through the I/O system.