This extension lets you redefine any image level as the base level (or maximum level). This is useful, for example, if your application runs under certain time constraints, and you want to make it possible for the application to load as many levels of detail as possible but stop loading and continue processing, choosing from the available ones after a certain period of time has elapsed. Availability in that case does not depend on what's explicitly specified in the program but on what could be loaded in a specified time.
To set a new base (or maximum) level, call glTexParameteri(), glTexParemeterf(), glTexParameteriv(), or glTexParameterfv() and set