Sorry, I thought I covered their use, or at least their use as I see it.
Great for teaching that some things shouldn't be chased and defining borders of territory.
But if you intend to use the dog for hunting, teaching it that some animals are electrically charged, will be counterproductive.
At all times think: "How will the dog reason it got zapped", not how a human would reason it out.
One example: I knew a hunter whose dog wouldn't let go of the pheasants it retrieved. So to teach it to let go of the dead bird, he zapped it.
Dog logic told it that pheasants are electric, so it never picked up another one.
Seems like most of my tips are negative, as in " Don't do this". So let me say that I have used one with great succes to teach my present Sam ( last of a line of Sams!) that fallow deer are not to be chased. I could teach him not to chase hares, roe deer and cats, but something about those flocks of stupid grain fed fallow deer just drew him in. I only had to zap him hard twice and he completely ignored them ever after.