The Talmud in the beginning of Masechet Shabbat delineates the various "domains" that are relevant to the prohibition against carrying on Shabbat. This prohibition includes transferring objects from one domain to another, and also carrying within a public domain. The first prohibition means carrying an object from a private domain to a public domain, or vice versa. The second prohibition is carrying four Amot (approximately six feet) within a public domain. If a person on Shabbat lifts an object in a public domain, carries it four Amot, and then puts it down, he has violated a Torah prohibition, even though he did not transfer the object from one domain to another.
The Sages introduced the concept of a "Karmelit" – an object in a public domain which, on the level of Torah law, is included as part of the public domain, but which is considered a separate domain on the level of Rabbinic enactment. A classic example of a "Karmelit" is a bench situated in the public domain. If the bench is higher than three Tefahim (approximately 10 inches) off the ground, and larger than 4X4 Tefahim (approximately 13X13 inches), then it is considered a separate domain. It would thus be forbidden to transfer an object from the bench to the public domain, or from the public domain to the bench, on Shabbat. Even though on the level of Torah law the bench is considered part of the public domain, the Sages enacted a provision requiring that we treat it as a separate domain. Since the bench constitutes an obstruction that blocks people’s path as they walk through the public domain, it resembles a separate domain and must therefore be treated as such.
There is a discussion in the Talmud whether small objects can be considered a "Karmelit" if they obstruct people’s path for various reasons, despite their small size. For example, people do not walk over thorns strewn over the ground, or over excrement. One might therefore have considered such substances a "Karmelit" even though they are not three Tefahim high or measure 4X4 Tefahim in size. The Shulhan Aruch, however, follows the view of Rav Ashi, that an object does not qualify as a "Karmelit" unless it is three Tefahim high and is 4X4 Tefahim large. It would therefore be permissible on Shabbat to transfer an object onto a pile of thorns in a public domain, but it would be forbidden to transfer an object onto a park bench.
Summary: It is forbidden on the level of Torah law to carry an object from a private domain to a public domain, or vice versa, and to carry four Amot (approximately six feet) within a public domain. The Rabbis enacted that we must treat objects situated in a public domain – such as a bench – like a separate, private domain, if they are at least three Tefahim (approximately 10 inches) high and 4X4 Tefahim (approximately 13X13 inches) large.
