On this daf, we find a question regarding hilchos Shabbos and a prohibition during Shmittah.
The Sefer Zichronos writes that borer is one of the most complex melachos. The Ramchal explains that even very complicated constructs can be internalized and applied with relative ease when one breaks them down to their central principles and sees how their minutiae are an outgrowth of these rules.
To begin this simple analysis: Borer is forbidden since they would select when preparing the dyes for the curtains used in the Mishkon. Our sages revealed that eating is not borer, and we hold that there are three rules of permitted selecting that make what we do considered eating: 1) B’yad, with your hand. 2) M’yad, at hand, for immediate use. 3) Good from bad.
Each of these rules requires precise definitions. Which vessel transgresses borer? One can use a fork or spoon to select, even though using a vessel that is made to select is forbidden, since these are mostly used like one’s hands and are not specifically designed to select.
What exactly constitutes immediate use? Most authorities permit from before a meal, and it is a dispute precisely how long before.
Although we must take what we want from what we don’t so as not to transgress borer, our sages explain that peeling an orange or the like is an exception to this rule. Since the peel covers the orange, removing it with a knife or one’s hands is like dealing with the orange itself, not a separate undesired thing (Tal Oros).
Another question comes up regarding Shmittah. During Shmittah, one must not discard leftovers of Shmittah fruits in a garbage or the like. To avoid this, it is normal to have a special Shmittah place, usually a small receptacle set aside only for leftover Shmittah fruits. If these leftovers got mixed in with regular garbage on Shabbos, it is forbidden to separate them. The mitzvah to dispose properly of Shmittah leftovers does not override the prohibition of borer in this instance. One’s need to put the Shmittah leftovers in a special receptacle is not considered an immediate use for him that would allow him to separate the leftovers from the garbage (Hilchos Shabbos B’Shabbos).