Avid uses extra frames (handles) before (head) a cut and after (tail) a cut to use in transitions. So if you have two clips that are both say a 30 seconds long. If you drop both of these clips end to end on the timeline and try to drop in a 1 sec (30 frame) dissolve, avid will complain. The way around this it before adding the clip to the timeline, set your out point 15 frames from the end of the first clip and set the in point of the second clip 15 frames into the clip. Add both clips to the timeline and drop in the transition. Avid uses the 15 “extra” frames from the end of one clip and the beginning of the next to create this 1 second (30 frame)transition.
Always keep this in mind when creating animation sequences in other applications (such as Adobe After Effects) making sure to create more material at the beginning and end than you need.
Erik