Dreams are separate "universes" within the Dreamscape that inherit unique properties. Every dream interconnects with each other in different ways. For example, one door in one dream may lead to another dream, or travelling far enough in one dream will eventually lead you into another.
Dreams are classified by number (e.g. Dream 0, Dream 52, Dream 249). This does not usually have any significant correlation to anything about the dream, instead, it is just a way to quickly classify a dream. Typically, a dream will also have some sort of nickname, or title, because remembering specific numbers in casual conversation is typically hard.
Dreams go into two classifications - normal dreams, and sub-dreams. Normal dreams are classified by a whole, positive number, starting at 0. Meanwhile, sub-dreams have significant correlation to a specific dream and are classified by a decimal, positive number, where the whole part of the number is the dream it is connected to. So, if it was the first sub-dream to be connected to Dream 1, it would be "Dream 1.1", and if it was the third sub-dream to be connected to Dream 5, it would be "Dream 5.3".
There is also a Stability Level to each dream using the common Rating System. The lowest rating means that the dream is very stable, safe, good, calming, etcetera, and the dream is overall a secure environment to be in. The highest rating, on the other hand, means that the dream is very unstable - dangerous, distressful, etcetera. You will typically not want to be in the higher-rating dreams.