How do you manage performance document with multiple assignments?

Here's the study case:

In an organization there are multiple assignments of an employee and while creating the document we can see multiple assignment so how do we make sure we have selected the primary one as there is no key or flag assigned to it.

Moreover there's no eligiblity crieteria for primary assignment in performance category eligiblity.

Is anyone has come through such kind of query please help me with any workaround possible as it was already there in enhancement request and still it has not been delivered by oracle.