A baraita was quoted on daf 23b that taught that if an area larger than 500 square amot was a karpef she-mukaf ledira – it was walled off for living purposes, and therefore one is permitted carry in it – and then vegetables were planted in most of it, the area loses its status as a place where people live, and one can no longer carry there. If, however, trees were planted in it, it is still considered used for habitation and it retains its status of a karpef she-mukaf ledira, so people can continue carrying in it.
The Gemara (24a) brings a disagreement between Rav Nahman and Avimi about this case.
Rav Yehuda said that Avimi said: This is only if the trees were planted in rows [itztablaot], the customary manner of planting ornamental trees in a courtyard. But if they were arranged differently it is considered an orchard, which is not made for dwelling, and where it is prohibited to carry. But Rav Nahman said: This applies even if they were not planted in rows, as people commonly plant trees in any arrangement in the courtyards of their houses.
Rav Nahman believes that any trees that are planted would serve the purpose of showing that people still make regular use of the area. Rav Yehuda quotes Avimi as saying that planting trees will only allow the area to retain its status as a karpef she-mukaf ledira if the trees are planted in a formal way in rows – like itztabla’ot. The word itztabla’ot originates in Greek, and it means a stable for horses. In fact, the Talmud uses the word to mean that in a number of cases. In our Gemara the word is “borrowed” to mean something that is arranged in neat rows, like horses in their stable.
The Aruk explains that according to Avimi if the trees are not planted in neat rows, then it is more difficult for people to walk in the yard, and therefore the potential use of the yard for normal daily human activities is lessened, causing it to lose its status as a karpef she-mukaf ledira. Nevertheless, the halakha follows the opinion of Rav Nahman, both because of his stature as the leading sage of his generation, and because of the following story related by the Gemara:
Mar Yehuda went to visit Rav Huna bar Yehuda and saw that there were people carrying in a karpef that was planted with trees in a haphazard manner. He inquired “aren’t you concerned with the position taken by Avimi?” to which Rav Huna bar Yehuda responded “I follow the position of Rav Nahman.”
When the Gemara relates a story that supports a particular position, it is usually understood to indicate that that position is considered normative.