Potential host nations can apply to the BIE to hold a BIE-sanctioned Expo to celebrate some special event, to represent some concept, or for some other them. BIE currently uses three classes: universal, international, and specialized. Historically other classification systems have been used.
From 1933 (when the treaty came in to effect until 1976, there were 2 types of expos:
The second phase of categories was with the 1972 protocol - which came in to force after Expo'75. Under that system there were just 2 types, Universal (the categories being combined) and International (or specialized) - where at a universal expo each country could build its own pavilion, but buildings could be provided for poorer countries. At an international expo, the expo built the pavilions. Also, the theme at a universal expo was more large or open. Only Expo '92 in Seville and Expo 2000 in Hanover have been held in this category.
The new categories, the 1988 protocol, came into in the last couple of years. There are 2 types - registered and recognized. The registered expos are to be every 5 years starting in 2010 (Expo 2005 was registered under the 1972 protocol, but decided to follow the new system anyway). It is up to each expo if they will let each country build its own pavilion or not, but the BIE has let it be known that they wish that only the expos in the "10 years" (2010, 2020 etc...) have countries building their own pavilions and the ones in the "5s" that the expo should build the buildings. In between these registered expos, there will be one recognized expo - which can be held for only 3 months and be no bigger then 25ha. Expo 2004 (if it's held) will be the first of this type. Both Genoa Expo '92 and Taejon Expo '93 were held as "test runs" for this type.