The Sages restricted carrying from one private domain to another, unless the owners of each established an eiruv before Shabbat. In the Mishna on our daf which opens the ninth chapter of Massekhet Eiruvin, three opinions are brought regarding private areas that are not used on a regular basis:
Rabbi Meir rules that the roofs of all houses are considered a single domain. It is permitted to carry from one roof to another, even if the residents of the houses did not establish an eiruv between them, unless they are more than ten tefahim higher or lower than the roof of their neighbor.
The Hakhamim say that each roof is a private domain unto itself. It is permitted to carry from one to the other only if the residents of both houses established an eiruv.
Rabbi Shimon says that not only roofs, but other similar areas – such as enclosed yards, gardens, orchards, etc. – are all considered a single domain, and objects that were in them before Shabbat began can be carried freely from one to another. Things that were inside a house when Shabbat began cannot be carried into another area, however.
Up until this point, when the Gemara discussed whether someone can carry from his house to a courtyard, or from a courtyard to an alleyway (mavoy – see 2a-b), the defining criteria was ownership. If the owners of all the houses in a courtyard agreed to make an eiruv, only then could they carry there. In examining the position of Rabbi Meir, the Gemara appears to suggest that, unlike carrying in and out of homes, in places like roofs, gardens, etc. the Sages were less concerned with ownership and more concerned with the definition of the type of area. According to this opinion, all roofs are one domain, regardless of to whom the roofs belong. Similarly, gardens, yards or orchards that neighbor one another will each be considered a single domain for carrying on Shabbat, even if they all have different owners. Nevertheless, according to Rabbi Meir, objects cannot be carried from one type of area to another (i.e. from the domain of roofs to the domain of gardens). Rabbi Shimon permits even carrying between different types of areas.