The Three Ma'arochos
"This is the Olah that burned (al mokdoh) on the Mizbei'ach all night" (6:2).
"and the fire of the Mizbei'ach shall burn it (tukad bo)" (Ibid.)
"and the fire on the Mizbei'ach shall burn on it (tukad bo), it shall not be extinguished" (6:5).
It is from these three words ("mokdoh" and twice "tukad") that the Gemara in Yuma learns the obligation of the Kohanim to make sure that there were three Ma'arochos (arrangements of wood) burning on the Mizbei'ach:
The Torah writes "al mokdoh" with reference to the main pile of wood that they lit in the middle of the Mizbei'ach, on which they burned the Korban Tamid and the other Korbanos.
It continues "al mokdoh" with reference to the second pile of wood, from which they took a light in order to burn the Ketores (on the Mizbei'ach ha'Ketores).
And it adds a second "tukad bo" with reference to a third pile of wood, which they kept burning to ensure that the fire on the Mizbei'ach burned continually. Consequently, if the fire on the large arrangement of wood showed signs of going out, they would rekindle it with this one.
The paragraph ends with another "tukad bo" - "An everlasting fire shall burn on the Mizbei'ach", from which Chazal learn that the fire for the Ketores had to come from the Mizbei'ach ho'Oloh.
And from the word "ve'ho'eish" (in Pasuk 5), they learn that Yom Kipur required a fourth arrangement, from which they took coals to carry, together with the special Ketores into the Kodesh Kodshim.
Bringing the Korban Tamid all Night
Strictly speaking, any Korban that is eligible to be brought at night-time, can be brought throughout the night until the morning.
The opening Mishnah in B'rachos teaches us however, that the Chachamim instated that they must be brought before midnight, for fear that one may fall asleep and the Korban will become Nosar ('leftover') thereby invalidating the respective Korban. Rashi explains there that the one exception to the rule is the fat-pieces and the limbs of the Korban Tamid, which were allowed to burn on the Mizbei'ach all night, and did not become Nosar at midnight (although the Rambam disagrees with Rashi).
The Torah Temimah suggests that the reason for this is based on the well-known Taz, who rules in Yoreh De'ah that whatever the Torah specifically permits, the Chachamim do not have the power to prohibit (by way of decree). And the Pasuk currently under discussion specifically permits the fat-pieces and the limbs of the Korban Tamid to burn all night.
Why Ma'ariv is Voluntary
It is obvious, that the Mitzvah of burning the remains of the evening Tamid (which primarily, the Pasuk is referring to) is not an obligation. It was only necessary with regards to parts of the Korban that had not yet burned up. In the event that they had burned by the time night-time arrived, the Mitzvah did not exist.
This explains why Tefilas Ma'ariv is voluntary (this may not be taken literally). The Chachamim instituted Shachris, Minchah and Ma'ariv corresponding to the morning Tamid, the afternoon Tamid and the burning of the fat-pieces and the limbs of the latter, respectively. Consequently, he explains, since the burning of the limbs and the fat-pieces is dispensable, so is Ma'ariv.
And by the same token, the author writes, it may well be that the Chachamim did not restrict Ma'ariv to before midnight (though it is certainly preferable to do so), because the Halachah is that the fat-pieces and the limbs may remain on the Mizbei'ach all night (like the opinion of Rashi that cited earlier).
