Examining the subitem, P-2, is the area depicted in that photograph familiar to you?
late 14c. (adv.) "moreover, in addition," from Latin item (adv.) "likewise, just so, moreover," used to introduce a new fact or statement, probably from ita "thus," id "it" (see id) + adverbial ending -tem (cf. idem "the same"). Thus "a statement or maxim" (of the kind formerly introduced by the word item), first recorded 1560s. Meaning "detail of information" (especially in a newspaper) is from 1819; item "sexually linked unmarried couple" is 1970, probably from notion of being an item in the gossip columns. Noun sense of "an article of any kind" (1570s) developed from adverbial sense of "moreover, in addition," which was used before every article in a list (such as an inventory or bill).