The Y axis is the Estimated Disk Activity; it is the average calculated disk service time divided by the duration and normalized to a 0 to 100 scale. In extreme cases, the estimated disk activity might be higher than 100; this is due to some assumptions that the Optimizer algorithm takes such as the random distribution, independent I/Os, and interleave mirror policy to serve reads.
Figure 9. Analyze Performance and Schedule
EMC Symmetrix Optimizer A Detailed Review