The linked CCI is display only and cannot be encoded on the decision. When you link an object version (Programme, Partnership Agreement, Accounts Decision, …) to a Commission Decision, then this object version will appear under "Linked To" on the Commission Decision.
The "Covers" list on a Commission Decision can be encoded on the Commission Decision and contains the CCIs the decision refers to.
For a Performance Review Decision, the EC user should fill in the "Covers" part. The "Linked To" part will normally stay empty because as a result of this Performance Review Decision, one or more Programme could have to modify their Priority Plan, which then would require a new Commission Decision.