What is the definition of "Mikreh Laylah"? What is the significance of "Laylah"?
Rashi and Targum Yonasan: "Mikreh Laylah" is an emission that occurred at night-time (not to preclude a day emission, but), because it is something that occurs more commonly at nighttime.
Which camp is the Torah instructing a soldier who has an emission to leave?
Rashi (citing Pesachim, 88a): With reference to wartime - when the Aron accompanied Yisrael into war - the Torah is ordering a soldier who had an emission, via an Asei and a Lo Sa'aseh, to leave the Machaneh Shechinah and the Machaneh Leviyah. 1
Ramban, Targum Onkelos and Targum Yonasan: It is instructing a soldier who has an emission to leave the camp of Yisrael during wartime. 2
Rosh: The Torah hints that if Yisrael become Tamei through idolatry, which is Metamei like a Nidah, they will be exiled to Bavel, and not enter the Machaneh (return to Eretz Yisrael) until the day of redemption.
Since the Torah writes "Machaneh" twice. See Sifsei Chachamim.
Why is a person who has an emission obligated to leave the camp?
Ramban: Refer to 23:11:2:2.