The Mechanism and Scope of Shinui Makom
The Rishonim dispute the nature of the mechanism of shinui makom. Rambam, Hilchot Berachot 4:3, is of the opinion that a change of location serves as an objective cessation of the meal. Anything eaten subsequent to the shinui makom has no relevance to the food eaten prior to the shinui makom. Therefore, if one changes locations and wishes to continue eating, he must first recite a beracha acharona on the original food, and then recite a new beracha rishona on the food that he wishes to eat. However, Tosafot, Pesachim 101b, s.v. K'shehen, are of the opinion that shinui makom is not considered a cessation of the meal. Rather, it is considered a hesech hada'at, an interruption of thought. While the food item that the individual plans to eat is considered part of the original meal, the hesech hada'at nullifies the original beracha rishona. Therefore, one is only required to recite a new beracha rishona. The beracha acharona that is recited upon conclusion of the meal covers the items that were eaten both before and after the change of location. Shulchan Aruch, Orach Chaim 178:1, rules in accordance with the opinion of Rambam. Rama Orach Chaim 178:2, rules in accordance with the opinion of Tosafot.
The Gemara, ibid, states that shinui makom only applies if one moves m'bayit l'bayit (from house to house). However, if one moves m'makom l'makom (from place to place) it is not considered a shinui makom. Rashi, ad loc., s.v. M'Makom, states that m'makom l'makom includes moving from the main floor to the attic. It is implicit from Rashi's interpretation that m'bayit l'bayit means that one moves from one building to the next. In contrast, Tosafot, ad loc., s.v. Ela, assume that m'makom l'makom is limited to moving from one corner of the room to another. M'bayit l'bayit includes moving from one room to the next. Shulchan Aruch, ibid, rules in accordance with the opinion of Tosafot that if one moves from one room to the next it is considered a shinui makom.
Nevertheless, Rama, ad loc., offers a compromise to the dispute between Rashi and Tosafot. There is a concept regarding Kiddush on Shabbat that is defined by location. The Gemara, Pesachim 101a, states that Kiddush must be followed by a meal in the same location (ain kiddush ela b'makom seudah). Tosafot, Pesachim 100b, s.v. Yedei Kiddush, note that although two different rooms in one building are considered two different locations, if one recites Kiddush with intent to move to a different room, one can consider both rooms as one location provided that both rooms are in the same building. Rama applies this leniency to shinui makom and states that if one recites a beracha with intent to move to a different location in the same building, it is not considered a shinui makom.
Mishna Berurah, Biur Halacha 178:2, s.v. B'Bayit, notes that although Shulchan Aruch, as well as Rama, rule that moving from one room to the next constitutes a shinui makom (when there was no prior intent to relocate to that room), there are many Rishonim who rule in accordance with Rashi that moving from one room to another room does not consititute a shinui makom. Furthermore, common practice is to move from room to room without reciting a new beracha. Mishna Berurah concludes that ideally one should not move from room to room unless he had intent to relocate at the time of the recitation of the beracha. However, since common practice is to be lenient on this matter, if one does move to another room, one should not recite a new beracha when he continues eating. Moreover, if one can see the original location from the new location, one may certainly move to that location.
Exceptions to the Rule
The Gemara, Pesachim 101b, mentions two possible exceptions to the principle of shinui makom. First, the Gemara quotes a Beraita that if one is eating with another person and then relocates while leaving the other person behind, this does not constitute a shinui makom. Ran, Pesachim 20b, s.v. V'Ta'ama, explains that by leaving an individual behind, when one returns, he returns to his original meal. Second, Rav Chisdah is of the opinion that if one eats a food item that requires him to recite the beracha acharona in the same place that he ate, shinui makom is not applied. Rav Sheshet disagrees and maintains that shinui makom is applied. Apparently Rav Chisdah is of the opinion that just as a "social obligation" to return to the meal does not produce a shinui makom, so too a halachic obligation to return does not produce a shinui makom. Rav Sheshet is of the opinion that only a social obligation can prevent a shinui makom. Shulchan Aruch, Orach Chaim 178:2, rules in accordance with the opinion of Rav Sheshet. Rama, ad loc., rules in accordance with the opinion of Rav Chisdah. [See Shulchan Aruch, Orach Chaim 178:5, and Mishna Berurah 178:44, regarding which food items this applies to. All opinions agree that it applies to a meal where bread is eaten.]
Although both exceptions to the rule are based on the same logic, they operate under different parameters. Rama, ibid, rules that if one employs the "halachic obligation" leniency to change locations, he may eat in the new location without reciting a new beracha. Magen Avraham 178:7, notes that this ruling is limited to the "halachic obligation" leniency. This is because when one eats in the new location, he recites the beracha acharona in that location (see Shulchan Aruch, Orach Chaim 178:4). Therefore, the food that he eats is a continuation of the original meal. However, if one is relying on the "social obligation" leniency, one who chooses to eat before returning to those he left behind is considered to have started a new meal that has no relevance to the original meal that he ate. He would then be required to recite a new beracha.
Eating While Traveling
Hagahot Semak 151:8, writes that travelers may eat while traveling and are not required to recite a new beracha at every location. This ruling is codified by Shulchan Aruch, Orach Chaim 178:4. Magen Avraham 178:11, explains that since there is no established meal from the outset, the change of location causes no significant interruption of the meal. Ostensibly, this leniency should be limited to a situation where the original beracha is recited in an unestablished location (i.e. an open area). This is evident from the ruling of Shulchan Aruch, Orach Chaim 178:3, that one who is in a garden may recite one beracha on all of the fruits in that garden. If he wishes to eat the fruits in another garden, he must recite a new beracha. Apparently, since the garden is enclosed, one cannot apply the leniency of the traveler.
R. Moshe Feinstein, Igrot Moshe, Orach Chaim 2:57, notes that there are situations where a traveler may start eating in his house without having to recite a new beracha on the road. First, if there is nothing compelling the traveler to remain in the house, he is considered to have already started his trip. In such an instance, he is not required to recite a new beracha upon exiting the house. If however, there is something compelling him to remain in the house (i.e., he is not ready to leave), he must recite a new beracha upon exiting the house. This is because when he starts eating he is considered established in the house.
Second, Chayei Adam 59:10, rules that if one is eating while walking, each bite of food is considered its own entity and upon relocating, a new beracha must be recited. This ruling is cited by Mishna Berurah 178:39. R. Feinstein notes that Chayei Adam's ruling only applies if one waits more than a few seconds in between bites. If one does not stop eating between bites, there is no shinui makom. Therefore, R. Feinstein rules that if one is sucking on candy or chewing gum in the house, and wishes to exit the house, one is not required to recite a new beracha upon exiting the house, even in a situation where one was compelled to remain in the house when he recited the beracha.