For example, a person might be upset because of a perceived "betrayal", or because of the "suddenness" of some action, or from being "stopped". There is no finite meaning to any of those words, it is completely a subjective thing. But certain keys will be important to a certain person. Some keys might be important in general to the person, and other keys might be central to particular issues for that person. Like, "my father never APPRECIATED me". We don't really know what exactly that means (at least not unless we ask for specifics and perceptions and so forth) but sometimes it might be enough to validate that that particular key is important to the person. We might have a dialogue and the client might suddenly realize "Ah, I was never APPRECIATED, now I understand." That might unlock the whole issue.
Keys are somewhat elusive to define. It can be all kinds of things. It is specific qualities or actions that are important as symbols. It might be symbols for good things or bad things. They keys will tend to unlock and free up an issue, simply by being brought up and looked at . They can be regarded simply as different angles of an issue.
If the client stumbles on a key by herself, just validate it and allow her to fully get whatever it entails. If the key unlocks something, there is usually not much point in specifying what it really means.
You can use lists of keys as an aid in dialoguing. That puts less strain on you having to come up with good questions. You can simply pick the next key on the list and ask the client if it somehow applies. Well, you would construct a question around it, but that would be trivial when you have the list of keys. So you would ask "Regarding your work, was something maybe not appreciated?" If the client doesn't particularly respond to it, then you take another key from the list and ask a question with it. If the key does trigger something with the client, work with it while there is still some action there. Get what she has to say, talk about it, ask for other examples, or whatever.
Lists of keys are fairly arbitrary. However, if you construct your own, be aware that they should empower the person and not validate ideas too far off the track of where we are going. There are pre-suppositions in all questions you ask, so be sure that they are useful. Preferably they should lead towards increased unity, responsibility, and empowerment. Deciding that somebody is a "jerk" is not a very good key. Realizing that one made a "mistake" is much more useful. Because it can lead to learning and integration.