Can anyone explain the difference between a task status of "Ready" and "In progress"?

Summary

In OBI reports using "Workforce Performance - Performance Task Status Real Time" subject area the Performance Task Status can be Ready or In progress but I can't work out what the difference is.

Content

We have an OTBI analysis using the subject area "Workforce Performance - Performance Task Status Real Time" and one of the columns is Performance Task Status Details > Performance Task Status. This column has values of Not started, In progress, Ready, and Completed. But checking these statuses (Ready and In progress) I cannot see a difference when looking at the documents in the UI.

Can anyone explain what the difference is between these two task statuses?