SMI stands for Secondary Mass Index, which reflects the little micro-beads inside the racquet. It should also be noted that there were two versions of PK5G. The so called SMI version is the older/original version. That said, the later version also has those micro-beads inside, but doesn't have the wording of SMI on the paint job. The later version is still in production and available at TW. But a couple more editions named Ki-5 were release since then. Each has different variances of different weight and paint jobs.What does it mean? I'm interested but what is your email?