Use

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

1. A player looking at the entity and using the +use console command (this is normally done by pressing the e key).
2. An entity output that specifies the Use input, or not specifying any input.
3. Directly calling the C++ function.
4. If an entity without a use function is parented to one which does, it will passthrough the input to the parent.

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.