No, there isn't a fantastic description of the reports out there for 5.8 per-field. The next major version will allow better dissection of the reports, though, to see exactly what makes them tick.
On the report you reference, that field cites "Workload". This is the badge|worklaod attribute for the VM at the time of that peak cited.. giving you an overall idea of VM workload in comparison to the latency peak. Not the disk workload, the overall VM workload. If you want disk workload and peaks, run the data store access and peak report.
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