Unused Registrations are courses taken with a Failed, Repeat, or Incomplete status assigned. You can drill into the blue number of unused registrations to see which classes are allocated as such.
Unused Registrations does NOT capture completed courses from the Credits Satisfying column (aka TAR bucket) over the 120 degree/graduation requirement credits. The number of Unused Registrations + Credits Satisfying will not add up to the total courses taken. All classes taken will be listed in the APR's Major (and any other programs of study) requirement tables.
For example, suppose an upperclassman completes a course that does not count toward degree or graduation requirements or is over 120 credits. In that case, you will not see those courses displayed in Unused Registrations.