The 6.0L Power Stroke was introduced for the 2003 model year and offered alongside the 7.3L Power Stroke for this single model year. By 2004, the 7.3L was completely phased out in favor of International's new, highly advanced 6.0L Power Stroke.
The 6.4L Power Stroke was introduced for the 2008 model year. Strict emissions regulations and the overall questionable reliability of the 6.0L were primary factors that presented a need for a new, clean slate engine. Quieter and cleaner than its predecessors.
Development of the 6.9L IDI began several years before Ford and International began their relationship, which would last nearly 30 years. And neither Ford nor International knew at the time that this engine would eventually lay the foundation for one of the most successful diesels to ever be offered.
The legendary 7.3L Power Stroke was introduced mid-model year alongside a turbocharged and naturally aspirated version of the 7.3L IDI. One could argue that the 7.3L Power Stroke is roughly based on the design of the IDI.