Zettai Ryouiki/Analysis
Some costume and cosplay enthusiasts categorize zettai ryouiki into five "Grades", from E to A. Only the top two (A and B) are considered true Zettai Ryouiki and they are only usable by females. Some researchers additionally introduce Grade S, the highest one which consists of Grade A plus further requirements to personality and hairstyle. Further reading on the grades here on Anime Deshou Deshou.
As a relatively recent phenomenon, the symbolism of Zettai Ryouiki only recently began to crystallize (or, rather, become apparent). Since it is incredibly sensual without revealing the "naughty bits", it symbolizes budding, innocent sexuality, one of The Ingenue who is already aware of her attractiveness but not (yet) cynical enough to put it to use (which is why many a Tsundere wears Zettai Ryouiki). And that's Moe. And Serious Business. Of course, as with any symbols, Genre Savvy characters can invoke it on purpose even if they're far from the described archetype themselves.
This academic treatise explains the appeal of Zettai Ryouiki as part of a more universal appeal of half-hidden beauty (also exemplified in such classics as a half-unbuttoned shirt, a semi-transparent laced dress, etc.). A revealing glimpse combined with a conservative cover-up of the other captivating parts is a classical bait for the curiousness and a powerful incentive for fantasy, which, applied to a girl, inevitably turns into attraction or, in more extreme cases, lust.
Zettai Ryouiki is currently the most popular fetish in Japan [1]. They are also a very popular fashion item with girls and young women in Japan, so the occurrence of Zettai Ryouiki can be regarded Truth in Television.