Um, not sure if you're joking or something, or genuinely don't know, so I'll tell you, but please don't think I'm patronizing you.
The 24 was the developmental prototype of many of the new technologies used in the next generation of variable fighters, such as the ICS, use of the EX-Gear, the brand-new FF-3001 thermonuclear engines, new materials, and so on. It was designed and built by Shinsei Industries on Earth I believe, and its flight test data and blueprints sent to Macross Frontier and Galaxy, where the data was used to help develop the VF-25 and -27, respectively. Of all the designs, the 25 was thought to have the most promise, based upon its flexibility in accommodating new mission roles and armor packs as its performance. The 27 was harder to build and not as flexible, and the YF-24 remained a prototype.
Hope that helps, and if I'm wrong, corrections are welcomed.