Candidate Search: difference between Employee and Ex-Employee in case of "Termination"

Hi team!
I want to better understand the reason why some candidates in candidate search are listed as "Employee", so Internal Candidate, and others as "Ex-Employee", so External Candidate.

In any case, I provided a termination on the Work Relationship and run:
- Load Worker in the Candidate Table
- Maintain candidates and job requisition for search

Someone can assist me?
It is important to me to understand because I have to manage a double massive loading concerning "ex-employee".

Please, share your experience
Thanks a lot in advance!
Tommaso