For all the JCK tasks is is required to provide the coverage numbers as part of completion summaries.
It is also very helpful to track current coverage as a regularly gathered metric during working on the task.
However a particular JavaSE feature may not represent a separate new java package.
Usually the feature is represented as a set of new classes and methods spread across the whole JavaSE API.
This makes if very difficult to track the coverage progress precisely for a particular feature using current APICov capabilities.

The request for an APICov feature: provide coverage for a difference between two signature files.

This will allow for example generate a sigfile for j.u.concurrent package for SE7 and SE8 and then collect the coverage telling how good are the updates to JSR166:Concurrency are covered in the JCK8.

This feature is not being requested for JCK8 but rather for JCK9.