The Shabbat has four domains [which define the limits of permitted and forbidden carrying]: the private domain and the public domain.
The Shabbat exitings [which define liability for prohibited carrying] are two, which are four.
Rava said, "The Mishnah means 'domains' [when it teaches 'exitings']: The Shabbat domains are two."
The Tosefta above is an odd text. It tells us that there are four Shabbat domains. We are prohibited from carrying from one type of domain to another on Shabbat. For instance, we may not carry anything of significance from our house (a "private domain") to a major street (a "public domain") on Shabbat. So far, so good. The odd thing here is that the Tosefta seems to provide only two of its four domains. Are there not two more domains that the Tosefta omits? When the Talmud (Shabbat 6a) quotes this Tosefta it "corrects" this omission, adding two more domains: the semipublic space (karmalit), and the space that is too small to be of significance (maqom patur). I would like to entertain the possibility that the Tosefta needs no correction—that it knows very well what it is trying to convey. To do this, we must look at our mishnah.
The phrase "Shabbat exitings" sound as odd in Hebrew as it does in English. Rashi (the foremost medieval commentator on the Talmud) understands this mishnah to be referring to the act of going from one domain to another while carrying something of value, thus violating the prohibition. Apparently, there are two major ways of violating such a prohibition, and each of these is divided into two sub-prohibitions. (Let's leave to the side what these four prohibited acts might be, for now.) The problem with Rashi's explanation is that yitziyot does not mean "carrying to the out of doors." That idea would be conveyed by the Hebrew word hotza'ot (literally, taking out). If we maintain that Rashi's explanation does not hold up to scrutiny, then what are these exitings?
Rava (a mid-fourth-century Sage) argues that exiting in the Mishnah is really a synonym for domain. In other words, he claims that our mishnah means to say, "We have a total of two Shabbat domains: the public and the private. However, each of the domains is divided into two sub-categories, yielding a total of four domains." Indeed, the Tosefta goes on to describe two private domains (one with barriers that rise up from the ground, the other a dugout into the ground) and two public domains (one, an open square, the other, a street). Here, we may see the Mishnah attempting to explain the oddness of the Tosefta.