Hey Ken,
Good question.
I think the term PoE cable is what may be confusing here.
Let's limit this discussion to Cat5e and Cat6 cable types.
In this case, there is no 'PoE cable'. Whether or not power is provided along with data is dependent upon the equipment the cable is connected to, not the cable itself.
As an example of this, let's say you run a Cat6 patch cable from a non-PoE switch to a camera. That cable is carrying data only at this point. If you take that same exact cable and connect it to a PoE switch and the camera (assuming the camera is PoE capable also), the cable will now carry data and power by nature of being connected to a PoE switch.
So let's say you want to use an injector. In this case, commonly you will have a non-PoE switch connected to a PoE injector (data only from switch to injector). You then plug a cable into the 'Out' side of the PoE injector and the other side into a camera. This cable is now carrying power and data from the injector to the camera.
The purpose of an injector is to provide PoE power to a device (i.e. camera) that would otherwise connect to a non-PoE switch.
Does this help clarify?