What do +,- and ? symbols represent in `service --status-all` represent in ubuntu 14.04

OBJECTIVE --- REMOVE/DISABLE USELESS DAEMONS.

What do +,- and ? symbols represent in service --status-all represent in Ubuntu 14.04. I am not able to understand it properly.

I guess + here might mean delayed services which start after the system startup.

Similarly, - would mean services running at startup.

No idea about the ? symbol. Somebody please help me identifying symbols.

Other options to let me know to disable useless services/daemons in Ubuntu 14.04 will be most appreciated.