As an example of the principle that a person should be careful to conform to the local custom and avoid disagreements, the first Mishna in our perek (50b) brings a case of consuming fruit on the Sabbatical year. The Mishna rules that if someone travels from a place where a certain type of fruit is available to a place where it is no longer available (or vice versa), he should behave according to the local custom.
Based on the passage in Vayikra 25:7, the Mishna in Massekhet Shevi’it rules that a person is allowed to harvest and store fruits that grow on the Sabbatical year as long as similar fruits are available in the fields for all. Once the season comes to an end and that type of fruit is no longer on the trees, the person who is storing the fruit is obligated to perform bi’ur (removal).
There are two main positions in the rishonim with regard to defining bi’ur during the Shemitta year. According to Rashi, the Rambam and the Ra’avad, once a certain type of fruit is no longer readily available in the fields, all such fruit must be destroyed. The Ramban and Tosafot rule that performing bi’ur means that someone who is storing such fruit must remove it from his house and make it hefker, i.e. declare it ownerless and available to all (according to some opinions only the poor would be permitted to make use of it).
In explanation of our Mishna, the Gemara on our daf quotes a Mishna from Massekhet Shevi’it (9:2) which teaches that not all places in Israel will end their seasons at the same time, thus someone could find himself traveling from Yehuda to the Galil, for example, and discover that his fruit, which was totally permissible to eat back home needs bi’ur performed on it in the new location. According to the Mishna there were three distinct areas in Israel: Yehuda (Judea), the Galil (Galilee) and Ever ha-Yarden (Transjordan). These places were established based on the Jewish population centers in the time of the Mishna, and areas whose population was mainly non-Jewish are not included.