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
2. An entity firing it through 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.