1. The Gemara discusses cases in which the people of the ground floor of the Chatzer and the people of the second floor may not jointly use an area adjacent to their respective domains.
2. The Gemara attempts to determine which is an easier form of usage of an area: lowering down an item to the area, or throwing an item up to the area.
3. The Gemara discusses a well full of water in the Chatzer and which can be accessed by the residents of the second floor.
4. The Gemara discusses why both the case of a pit and the case of a rock are mentioned in the Mishnah if they have the same law.
5. The Beraisa discusses what should be done in general when a joint Eruv was not made between the residents of the two floors.
A BIT MORE
1. Any area that is equally accessible to both the first and second floors may not be included in the separate Eruvin of the first and second floors. The area may be used by the two areas together only when they have made a joint Eruv.
2. The Gemara earlier (83b) said that this is the subject of a dispute between Rav and Shmuel. Shmuel says that lowering is considered an easier usage.
3. The Gemara says that it may be used only if a joint Eruv was made. This is in contrast to a pit filled with untithed fruit that is considered like any pile of earth and is Muktzah.
4. The Gemara explains that one might have thought that the pit filled with untithed produce should not be considered like earth, since there may be times when it will contain tithed produce and the law will be different (i.e. more stringent). This is why the Mishnah needs to mention the leniency regarding the case of the pit.
5. The lower ten Tefachim are used by the residents of the ground floor, and the ten Tefachim closer to the top floor are used by the residents of the top floor.