There are four categories of “domain” with regard to carrying out on:
A private domain is an area larger than 4 by 4 handbreadths, divided from the surrounding area by a fence at least 10 handbreadths high. One is permitted to carry within the private domain, which is considered to extend upwards to the sky.
The public domain is a place at least 16 cubits wide, through which many people (some authorities say 600,000) pass daily. The public domain is considered to extend 10 handbreadths upwards. On one is prohibited to carry objects a distance of more than 4 cubits in the public domain. One is also prohibited to transfer objects to or from the public domain.
The third type of domain is an exempt place, which is an area less than 4 by 4 handbreadths that is separate from the surrounding area.
The Sages added a fourth domain, called a karmelit – an intermediate category of domain between a private domain and a public domain.
Some of these laws are exhibited in the following case that appears on today’s daf.
One who was reading a sacred book in scroll form on on an elevated, wide threshold and the book rolled from his hand outside and into the public domain, he may roll it back to himself, since one of its ends is still in his hand. However, if he was reading on top the roof, which is a full-fledged private domain, and the book rolled from his hand, as long as the edge of the book did not reach ten handbreadths above the public domain, the book is still in its own area, and he may roll it back to himself. However, once the book has reached within ten handbreadths above the public domain, he is prohibited to roll it back to himself. In that case, he may only turn it over onto the side with writing, so that the writing of the book should face down and should not be exposed and degraded.
In the case of a person on a threshold who was reading a sacred text written on a scroll and that scroll unrolled and landed on a karmelit (Mishna Berura), if one end of the scroll remained in his hand, he may roll it back to him. That is the ruling even if the threshold was a private domain, i.e., four by four handbreadths and ten handbreadths high, and the scroll unrolled into a public domain. This was permitted in order to prevent disrespect for the sacred text. However, if the book fell from his hand completely, he is permitted to roll it back only if it rolled into a karmelit.