What is a "special" (18) and how does the idea of a "special" widen the scope of the reader's understanding of Trash's setting?

A "special" (18) is what the dumpsite boys call a bag of trash that is intact and has come from the wealthier neighborhoods surrounding Behala. Raphael reveals that in those wealthier neighborhoods, there are toilets, so in addition to being more likely to find valuable items inside a special, they are less likely to find human waste to impede their search efforts. This information lets the reader know that there is indeed a socioeconomic hierarchy within Andy Mulligan's unnamed country. The dumpsite boys are just not fortunate enough to be a part of the higher strata.