Continued from part II
Given that a house can have more than one door and still be classified as a reshus hayachid, an area totally enclosed by doorways would be considered a reshus hayachid as well. In effect, a tzuras hapesach, two posts one on either side of the opening with a crossbeam [or a string] running across the top of them, forms the doorway[s] necessary for an area to be termed a reshus hayachid (Eruvin, 11b and Shulchan Aruch, O.C. 362:11). 
A tzuras hapesach would reclassify a karmelis as a reshus hayachid. However, regarding a reshus harabbim, the Shulchan Aruch (O.C. 364:2) states that only delasos [doors to close the breaches] would rectify it and not a tzuras hapesach. Therefore, it is essential to establish the classification of an area ― is it or is it not a reshus harabbim? ― in order to ascertain what kind of enclosure would reclassify the area as a reshus hayachid where it would then be permissible to carry.
 There are those who claim that the term eruv refers to eruvei chatzeiros and not to a tzuras hapesach. However the Gemara (Eruvin, 6a) calls a tzuras hapesach an eruv; see also the Piskei Rid (Shabbos 6a) for further proof that the term eruv applies to both the physical construct and the brachah.
 The Rambam considers a tzuras hapesach a valid mechitzah only when utilizing at the minimum two mechitzos that are omed merubeh al haparutz (Shulchan Aruch, O.C. 362:10). Where this is not the case, each pole can be no more than ten amos apart from the other. The Kaf HaChaim (362:92) quotes the sefer Minchas Yehudah (siman 26) that states if a city has omed merubeh of its houses, they can be used to qualify the tzuras hapesachim as a Rambam eruv. Since the proximity of property lots in Brooklyn is such that they are omed merubeh al haparutz ― particularly the fences that surround the property lots ― any eruv in Brooklyn would be considered a Rambam eruv. Additionally we have determined that Brooklyn is encircled with three mechitzos that are omed merubeh al haparutz (see Part 3: Why Brooklyn Is Not a Reshus HaRabbim); therefore, any eruv in Brooklyn would definitely be considered a Rambam eruv.
 Most poskim maintain, me’d’Oraysa, a tzuras hapesach would reclassify a reshus harabbim as a reshus hayachid; however, me’d’rabbanan there is a requirement of delasos (Rosh Yosef, Shabbos 6b; Shulchan Aruch HaRav, O.C. 364:4; Tzemach Tzedek, Eruvin the end of Perek 5; Aishel Avraham, siman 345; Gaon Yaakov, Eruvin 11a; Yeshuos Malko, O.C. 21; Aruch HaShulchan, O.C. 364:1, and Kaf HaChaim, O.C. 364:12; see also Part 1: Delasos – Me’d’Oraysa or Me’d’rabbanan). Since the requirement of delasos is me’d’rabbanan, we can be lenient [safek d’rabbanan l’kulla] and apply any additional heter to remove the requirement of delasos (Kanah V’Kanamon, 5:56; Livush Mordechai, 4:4, and Bais Av, 2:9:3).