Shabbat 6 - Four areas for Shabbat
One is not allowed to transfer objects from a private domain to a public domain and vice versa, on Shabbat. If he does so unwittingly, then, once he realizes his mistake, he has to bring a sacrifice. If he does it consciously and there are witnesses who tell him not to do it, he is liable to stoning, and if there are no witnesses, he may be cut off from his spiritual source. What is considered private domain for this purpose? One's house qualifies, but even any area bigger than four by four hand-breadths and surrounded by walls, or partitions, is also considered private. What is considered a public area? Any highway, a large public plaza, a street that is not delimited by doors from both sides, and any place where multitudes walk.
There are also areas that are not completely private - because they are not surrounded by walls - nor are they public, since multitudes don't usually go there. Such areas includes a sea and an area in front of a store where merchants show their wares. Since such an area has the traits of both private and public domains, the Sages prohibited carrying in them, just like in public domain, and transfering from them to both a private and a public domain.
Finally, there are areas that are so small and insignificant that were called "exempt," such as a threshold of a house. Because they are so small, one may transfer to them and from them, so long as he does not use it to remove the prohibition altogether, by passing the object first to the threshold and then into the house.
Art: Ferdinand Loyen Du Puigaudeau - Village Plaza