What is a control wire in a circuit

I wasn’t sure where to post this question; but I’m trying to understand, in general ,what is the function of a control wire vs. a power feed? For example say you have a 2-wire connector leading to an evap vent solenoid and one wire is a power feed and the other is a control wire. In this case what would the function of the control wire be? And to go a bit further how can you go about identifying a control wire? Thank you in advance

Just to take a step further, the control wire is responsible for completing the circuit. Identification can be as simple as disconnecting the component. The wire that changes voltage when unplugged is often the control wire, as is illustrated in Andy's post.

Thanks to you both for clarifying this for me.