The Mishna (23b) at the beginning of the perek taught that food for the eiruv can be used even if one of the people involved in the eiruv cannot eat it. For example, wine can be used for a Nazir who cannot drink wine. Similarly, teruma, which is permitted only to the kohen, can be used for an eiruv that includes non-kohanim. In both of these cases, the food itself is permitted food, but the individuals involved are not allowed to eat it.
The Gemara (30a) discusses whether food that someone has forbidden on himself by way of neder or shevua (if he swears not to eat the food) can be used for the eiruv. In the course of the Gemara’s discussion, two baraitot are quoted, each in the name of Rabbi Eliezer. According to the first baraita, if a person swears that he will not eat a specific loaf of bread, it still can be used for the eiruv; if, however he vows that a specific loaf is forbidden to him, then it cannot be used for the eiruv. According to the second baraita that is quoted, even saying that a specific loaf is forbidden will not prevent the loaf of bread from being used for the eiruv; the only case that will not work is if the person states that the loaf should be consecrated for the Temple. Unable to reconcile these two baraitot, the Gemara concludes that two students must have transmitted different versions of Rabbi Eliezer’s teachings.
Throughout the Talmud the difference between neder and shevua, as is noted in our Gemara, is that a neder refers to the object – that the thing itself is now forbidden – while a shevua refers to the person – that the person is now no longer permitted to partake of the object. This distinction becomes significant in cases like ours, where a forbidden object cannot be used for the eiruv, but a food that is not permitted to a given individual can be used – even for the person who is not allowed to eat it.