The third Mishnah in the sixth chapter of Maseches Gittin deals with two topics: the ability of a minor girl to appoint an agent to receive her get as compared with her father's ability to do so, and the law of a get for which a specific place of delivery was designated but which was not given there.
A minor girl who appoints an agent to receive her get:
"HaKetanah she'amrah hiskabel li giti - eino get ad sheyagia get l'yadah" - a girl who has not yet reached the age of mitzvos, who says to a person whom she wishes to appoint as her agent, "Receive my get for me," does not thereby make him an agent for receiving. As we learned in the previous Mishnah, a minor girl may receive her get herself, provided that she knows how to safeguard it; but she does not have the ability to appoint an agent to receive it on her behalf. Therefore, even though the agent takes the get and hands it to her, the get does not take effect until it actually reaches her hand.
"Lefichach im ratzah haba'al lachzor bo - yachzor" - as long as the get has not reached her hand she is not divorced, and the husband may retract. The reason for all of this is stated explicitly in the Mishnah: "she'ein katan oseh shaliach" - one who is not yet obligated in mitzvos is halachically incapable of appointing an agent.
Appointment of the agent by the father:
"Aval im amar lo aviha tzei v'kabel l'viti gitah" - the father is of full mental capacity and can appoint an agent, and since his daughter is a minor, he has the ability to receive her get on her behalf. Therefore "im ratzah lachzor bo - lo yachzor": once the get has reached the hand of the agent, she is divorced, and the husband can no longer retract.
Designating a place by the husband:
"Ha'omer ten get zeh l'ishti b'makom peloni, v'natan lah b'makom acher - pasul" - the husband wants the divorce to take effect specifically in a particular place, and if the agent did not heed his words and handed over the get elsewhere, it is not a get. The reason: the husband is particular about this, and he appointed the agent only on condition that he carry out his instructions - perhaps he is uncomfortable with the matter becoming known to the people of another place, or something similar. It emerges that the agent himself nullified his agency by delivering the get in the wrong place.
"Harei hi b'makom peloni, v'natan lah b'makom acher - kasher" - here the husband is not making the place of delivery a condition, but is merely informing the agent where she is to be found. Conveying information about her whereabouts is not an indication that he is particular about the place where the divorce takes effect, and therefore the get is valid.
Designating a place by the wife:
"Ha'ishah she'amrah hiskabel li giti b'makom peloni, v'kiblo lah b'makom acher - pasul" - according to the first Tanna, her law is like that of the husband: she too is particular about the place of receipt, and since the agent did not receive the get in the designated place, the get is invalid.
"V'Rabbi Eliezer machshir" - Rabbi Eliezer disagrees regarding the wife, and does not view her particularity as grounds for invalidating the get. The reason: since she can be divorced against her will, this is not a matter about which she has the standing to be particular.
"Havei li giti mimakom peloni, v'hevi'u lah mimakom acher - kasher" - here she is not appointing an agent to receive the get but an agent to bring it, and the get does not take effect until it reaches her hand. Therefore the get is valid even according to the first Tanna, who holds that the wife is particular, since her particularity applies only to the place where the divorce takes effect. Had this been an agent for receiving, the get would have taken effect at the place of receipt, and there she would indeed have grounds to be particular; but since the get takes effect only when it reaches her hand in her own location, it makes no difference that she said to bring it from a particular place.
In summary: in this Mishnah we learned that a minor girl receives her get herself but cannot appoint an agent, and therefore the husband may retract until the get reaches her hand, whereas her father can appoint an agent to receive it, and once the agent has received it she is divorced. We also examined the distinction between a husband who designates the place where the get is to be given, whose particularity invalidates the get, and a husband who merely informs the agent where his wife is to be found, in which case the get is valid; the dispute between the first Tanna and Rabbi Eliezer regarding the wife's particularity about the place of receipt; and the fact that with an agent for bringing the get, all agree that the get is valid.