The primary goal of the design of this format is to ensure that the map can be displayed quickly. This is achieved by using a relatively compact binary format and allowing the map information to be divided up into areas. The areas are rectangular - at least on a Mercator projection - and bounded by lines of latitude and longitude. It is a quick operation at display time to see if an area overlaps the screen window - if it does then each of the items in the area is examined to see if it needs to be displayed, if it does not then all of the items in the area can be skipped.
For example, the format currently classifies objects into line features and named point features. Lines are, in turn, classified into coast lines, airspace boundary lines and ground feature lines. In future, additional sub-classes of ground feature line such as road, river, runway, airfield boundary, etc could be added. An old version of EWView would then display these as generic ground features.
However, because of time constraints on the original implementation, the hierarchy information is not yet stored in the file format so new feature classes will not be recognized by old software.