Is there a difference between a Swing Key and a Clear Key?
The short answer is "a lot."
A Swing Key typically is thought to be, and used as, (by most players) a "trigger" for the swing. A swing key also serves some players as a short-term reminder of something they want to be sure to remember on each shot. It's like taking a grocery list to the store so you don't forget something for dinner. It's like a string tied around the finger. Or, for some players, it may be similar to carrying a rabbit's foot or other "charm" as a safeguard against "bad Luck." As such the swing key tends to possess a ritualistic nature. Now that's not strange when you realize that all ritual has an element of purposefulness as a human barrier against things considered to be "evil." When a swing key is only used in such ways, however, it has not been tapped for its full potential. That is only one small part of a much larger set of issues.
Using a swing key fully means seeing that it has a valid role in the architecture and structuring of learning. Using a swing key is also an asset in giving leverage to the necessary pre-shot command (supervision, direction) important to the execution of an upcoming shot, that is if we prefer not to leave the shot to whatever our non-discriminatory systems may bring up from default.
Both of those functions are manual, however, and involve only conscious thinking. It is, after all, necessary to think about a skill in order to put it into action. And, again because of the non-discriminatory element in every person's system, it is necessary to inform oneself directly, before execution (not during it) about which already formed habit is desired for the shot at hand. Bear in mind that habit execution is quite different from what is involved in executing a skill. One must be explicit in instructing one's own system before walking to the ball to make a shot, and that means knowing how that non-discriminatory human system goes about its "business."
All of that is, of course, only important if what is desired is a consistent and reliable playing framework. Swing keys can help in learning and shot-planning. Once an instruction has been given in pre-shot, however, we have to be ready to let go of the swing key and move to a Clear Key. That is, if what we want is to reach the automatic level for execution. For learning, swing keys are valuable to the necessary instruction to create a skilled learning response. Then we need to follow with Clear Keys in order to facilitate moving the skill to the habit level, when learning, or simply going directly to the habit level, when playing.