The third type of domain is a mekom petur (an exempt area). According to Torah law, this includes fields, deserts, oceans, lakes, and other places not enclosed by walls (and thus not deemed reshut ha-yaĥid) and also not used by the masses on a regular basis (and thus are not deemed reshut ha-rabim). Since these are undefined places, they have no significance as a location. An object in a mekom petur is not located in a place that establishes a connection with it. Therefore, Torah law permits carrying an object from a mekom petur to a reshut ha-yaĥid or a reshut ha-rabim, and vice versa. Similarly, one may carry objects within a mekom petur for as long and far as he wishes.
However, since a mekom petur is in some ways similar to a reshut ha-rabim – as the masses may make use of both types of areas – the Sages safeguarded the Torah and declared that any open area that is not a reshut ha-yaĥid shall be called a karmelit and have a status akin to that of a reshut ha-rabim. Thus it is prohibited to carry an object more than four amot in a karmelit, and one may not carry anything from a karmelit to a reshut ha-yaĥid or reshut ha-rabim, and vice versa.
The only places that are still called a mekom petur are those that are not fit for significant use, such as rocks that are higher than three tefaĥim (about 23 cm) and also less than four tefaĥim wide (about 30 cm). One may carry something from a mekom petur into a reshut ha-yaĥid or reshut ha-rabim, and vice versa. Such a mekom petur was not included in the decree because it is fundamentally different from the other domains. Since it is over three tefaĥim high, it is distinct from the ground, and since it is less than four tefaĥim wide, it is not big enough to be significant. Therefore, no one will make the mistake of thinking that if one may carry in an insignificant mekom petur, one may also carry in a larger and more significant place.
. A mekom petur can be within a reshut ha-rabim. The poskim disagree whether a mekom petur can be within a karmelit. Some say that since a karmelit started out as a mekom petur but the Sages categorized it as a karmelit, any mekom petur within it is subsumed by its karmelit status, and is treated as a karmelit (Ran, Hagahot Maimoniyot, Tur, and Beit Yosef quoting Rambam). Others maintain that since a karmelit has the status of reshut ha-rabim, if there is an area within it that is more than three tefaĥim high and less than four tefaĥim wide, that area is a mekom petur (Rashi, Magid Mishneh quoting Rashba, Me’iri, and Rabbeinu Yeruĥam). But, within a reshut ha-yaĥid, all agree there cannot be a mekom petur, because the fence that surrounds the reshut ha-yaĥid makes everything within it part of the reshut ha-yaĥid (Rema 345:19; BHL s.v. “Ran” and “ve-yesh ĥolkim”).A reshut ha-rabim extends only to the height of ten tefaĥim. If one takes an object in hand and walks on a tightrope or beam that is set up above a reshut ha-rabim, he is not transgressing the prohibition of carrying, because the space above ten tefaĥim is considered a mekom petur. However, there is a disagreement regarding the law relating to a table that is over ten tefaĥim high and four tefaĥim wide that is standing in a reshut ha-rabim or a karmelit. According to SA 345:16, since there are no walls to turn it into a reshut ha-yaĥid, it is considered a karmelit, while according to MB 345:66 and the Vilna Gaon (based on a number of Rishonim), it is defined as a mekom petur because there is no such thing as a karmelit or a reshut ha-rabim above ten tefaĥim. SHT ad loc. 68 states that R. Yosef Karo recanted (R. Elimelech Lange, Hilkhot Eruvin, p. 20).