That's a good question and although I am no authority on it, from what I understand dissociation is a continuum. It starts at a level that we all do a little of like driving down a familar route and not remember how you got from point A to point B while you allow your mind to drift into your thoughts rather than paying close attention to your driving. I think Shrinklady mentioned it is also when we get lost in a good book or engrossed in a movie that we sort of lose awareness of our surroundings and time. Everybody does this on occasion.
When we dissociate as a result of trauma that usually classifies as a disorder and can be acute or chronic. As a continuum it can slide down the scale and become a little more complex the further it goes. When it can be said to be a disorder seems like it is when it is causing interference in your life which can also vary in certain degrees. When it is uncontrollable and disrupts your life it is probably a disorder and frequency may play a part in that as well.
I'm not sure if you were looking for such a simplistic answer to your question. You may have been aware of this longer than I and probably know much more than I do on the subject. Right now I am containing my desire to learn too much about this until my T determines where I fit into the continuum. You can only imagine how difficult this is for an information junkie (Yes HB you're not the only one)to resist researching something that personally affects them.
As for the importance of a diagnosis I think that is important in regards to determining the best treatment plans and to help with support and integration, but it should not be our primary focus. We had an intersting discussion on diagnosis. I don't know if you seen that:do you think having a diagnosis is good...
I don't know if any of this helps, but whatever we learn here is only a supplement to our therapy. So trust in whatever your T (T's) believe is best for you.
JM