That is probably hard to explain. I may not really understand it too. It asks of the person's ability to recall and of the stages of sleep related to brain activity. Dreaming occurs in stage 5 known as REM sleep. It is naturally followed by returning to stage two.
Pretty much if you awaken in stage 5 you will have a higher probability of dream recall (Portion) as you were actually dreaming at that point. But, since we awaken kinda' progressively only that portion of dreaming at the edge of awareness between consciousness and sub-consciousness will be recalled. When fully conscious you are now aware of environment and have usage of working/short term memory or only fragments of the dream that were imprinted.
If awakened naturally moving from stage 5 (1 hour) to stage two (lasts 20 minutes?) you can say the dream is long gone and now buried in long term memory. To be 'easily' recalled from memory it needs to be imprinted in some way. Dreams are not really imprinted since there really is no conscious awareness. They are experienced.