This optional class allows the author to 'reserve' a specified amount of disk space in any directory, depending on the installation ...

This optional class allows the author to 'reserve' a specified amount of disk space in any directory, depending on the installation state of a component. Reserving cost in this way could be useful for authors who want to ensure that a minimum amount of disk space will be available after the installation is completed. For example, this disk space might be reserved for user documents, or for application files (such as index files) that are created only after the application is launched following installation. The ReserveCost class also allows custom actions to specify an approximate cost for any files, registry entries, or other items, that the custom action might install.