Different stimulation levels are what bring versatility to electronic training collars. The more levels that a collar has, the more you will be able to adjust the amount of correction given. Then, you can tailor the correction to what the situation requires, which can vary depending on temperament as well as from dog to dog.
Adjustability at the transmitter is another feature to look for. To change the intensity level on older collars, you had to get out a special wrench to change the contact points. Since this was a time consuming process, most owners simply put points in that were hot enough to make the dog yelp and left them in for the duration. This does not always render the collar as an effective training aid since the dog may receive more stimulation than the individual situation required.
Now, many models have the intensity level adjustable directly on the transmitter. I find this to be invaluable for several reasons. The main reason is that the same dog will require different levels of stimulation during different scenarios. For example, if my dog is calm, the 90-pound bruiser turns meek on a level 2 pulse. However, if he is on scent or really riled up, it takes level 4 to get his attention.
Not only is this more humane, since you can keep the stimulation down just to the level that gets your dog's attention, but it can also result in a safer situation in the field.
Continuous stimulation is the traditional method, delivering an electronic pulse for as long as the trainer depresses the button. Most models also have a timeout function set at seven to ten seconds.
Since it is a longer duration than a momentary pulse, the amount of stimulation felt by the dog is much stronger, even when set on the same level. This type of stimulation is excellent for use in a hunting situation. When my dog has a full nose of fresh bird scent and is excited it takes more than a tap to get his attention. He wants to go, and has an entire different demeanor than in a training situation. A quick momentary pulse is forgotten too soon, and the dog continues with the bad behavior. Therefore, most hunters use continuous stimulation to maintain control in situations with numerous distractions to get the dog back on track.