There are several tiers of international economy fares:

* Full Y (Y class) is very expensive, freely refundable and changeable, valid for stays of up to a year, and books into premium economy on a space-available basis (but doesn't confirm it IIRC).
* "Ecowari premium" (E class) is less expensive, confirms premium economy and is valid for stays of up to six months.
* "Ecowari premium light" (also E class) is the cheapest type of confirmed premium economy and is valid for stays of up to four months.
* "Ecowari standard" (B class) is the lowest freely-changeable Y fare and usually the lowest fare you can upgrade to biz with ANA miles or upgrade points.
* Then there are various "ecowari" discount economy fares (usually W, S, K or L class). The exact combination depends on the route--there are more tiers on long-haul routes and fewer tiers on regional routes. The main distinctions between these levels are how far in advance you book and whether your trip includes a Saturday night stay.
* Then you can book an ANA flight through another airline's fare structure (i.e. as a codeshare), in which case basically none of the above applies.