Can you help me understand the functionality of "PRocess Eligibility" in Check Eligibility section?

Summary:

My understanding is that "Process Eligibility" is to process eligibility for an employee adhoc, instead of running batch eligibility process for all employees.

Based on this assumption, I changed the Business unit of an employee and tried to process his eligibility for a Performance Document and Review Period selected from Check Eligibility section.

I entered the review period, and Performance template details on the check eligibility page , "Overall Eligibility status is showing as eligible" without running the batch process and "Process Eligibility" process and the "Process Eligibility button" is disabled now. why is this happening ?

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