(Question)
I've been confused by how I see people using "whoa" when training steadiness and how it should be used during hunting.

(Answer)
You've probably discovered that your dog has a fairly extensive vocabulary. The average dog is capable of learning about 160 words and some dogs will learn as many as 300. That's about the same number of words a young toddler knows! It's been my experience that dogs perform best when they have no confusion about what you are asking of them, so you should never use the same word to mean more than 1 thing. Just think how confusing it would be if you used the word, "sit," to mean both sit down and lie down! Similarly, you will confuse your dog if you use "whoa" to mean both stop and stay.

Like many trainers, I have 3 separate commands associated with stopping and remaining that I use to let my dogs know what is expected:

(1) WHOA - Stop moving and do not move again until released or given another command (preferably my dog should remain standing)
(2) WAIT - My dog is already stopped and she is to stay where she is until released or given another command. If she is standing, I prefer she remain standing and if she is sitting, I prefer she remain sitting.
(3) STAY - My dog is doing whatever (walking, running, sitting - doesn't matter) and she is to remain while I leave. She may move, but she may not follow me. (For example, I am leaving the yard and she is to stay there.)

Like all commands, I begin using these in the house as soon as I bring my puppy home. Practical examples of how I teach these commands are:

(1) My dog comes into the house and heads for the couch. I see she has muddy feet. I say "whoa" and she stops and remains standing there while I go get a towel to clean her up. After she's cleaned up, I give her the release command (OK, with a tap on the head) and she may get onto the couch.
(2) My dogs are at the door waiting for me to open it so they can go outside. I see that KD does not have her yard collar on. I say, "KD, wait," then I open the door and let the other dogs out. After I put on her collar, I give her the release command (OK, with a tap on the head) and open the door so she can go out.
(3) I am going to the grocery store. My dogs are hovering around me, near the door. I say, "Girls, stay," and I open the door and go out without them attempting to bolt out ahead of me.

While hunting, you might use these commands in these ways:

(1) My dog and I are hunting with a friend and his dog. His dog goes on point. My dog comes running toward the dog on point, but makes no signs of slowing down to stop and honor/back my friend's dog. I say "whoa" and my dog stops. My friend flushes the bird, shoots it and sends his dog for the retrieve. After the retrieve has been made, I give my dog the release command and we all continue hunting. 
(2) My dog goes on point on the other side of a fence. I must crawl over the fence in order to flush the bird. I don't want my dog to flush the bird until I'm over the fence and ready to shoot. I say, "Wait" and my dog remains still until I flush the bird and/or release her. This command is also used in the duck blind. I position my dog just outside the blind so she can mark the fall of the ducks as I shoot and tell her to "wait" so she does not swim out to make the retrieves until all the shooting is over. 
(3) I have shot a bird that has fallen near a road or other area that may be dangerous for my dog. I call my dog to me and heel her to near the location of the fallen bird. I say, "stay" and leave my dog while I go get the bird. When I return, I release her with another command, "hunt 'em up!" and we continue hunting.

What you will often see in a training or hunting scenario, is the "Whoa" command used improperly. The dog is on point. As the handler walks up to the dog to flush the bird, he repeatedly says, "Whoa," in a loud, stern voice. Obviously he is not trying to stop the dog - the dog is already stopped. What he is actually trying to do is *warn* the dog not to move. Hugh? Dogs live in the moment and do not understand warnings. Doing that is not only a waste of breath, but a waste of an opportunity to reinforce desired behavior. The handler should instead repeatedly say, "nice" to mark the continued moments the dog is staying steady (exhibiting the desired behavior). Because the "whoa" command implies stopping and not moving, an additional command is not needed. But, let's say you can tell from your dog's body language that he's probably going to move - then the proper command to use is "Wait," given one time, followed by repeated "nice" to mark the desired behavior that follows or "no" to verbally reprimand for failure to comply.