https://books.google.com.jm/books?id=cSAVDMiQQ5QC&pg=PA17&lp...
Artículo 8: Garantía "ad personam".
Se respeterán las situaciones personales que, con carácter global, excedan del convenio, manteniéndose estrictamente "ad personam" las que vengan implantadas por disposiciones legales o usos y costumbres, cuando examinadas en su conjunto resulten más beneficiosas para el trabajador. ...


https://www.oxfordreference.com/view/10.1093/acref/978019536...
Ad personam
To a person. Relating to an individual person.


https://www.proz.com/kudoz/italian-to-english/law-contracts/...
GLOSSARY ENTRY (DERIVED FROM QUESTION BELOW)
Italian term or phrase: ad personam
English translation: on an ad personam / personalised basis
on an ad personam / personalised basis
Explanation:
I have seen this expression used in some English texts too to refer to something that is tailor made or that refers to a specific person only. My suggestion would be to leave it as it is or, alternatively, use "on a personal/personalised basis". Below are some links that provide further info. Hope it helps.
"The higher salaries of staff already in the grades were to be maintained on an ad personam basis and to be adjusted by 1 percent for each 5 percent movement.."

Other suggestion which had 2 peers agreeing: "personally" (personal).


Spain: Wage formation | Eurofound https://www.eurofound.europa.eu › publications › report
Mar 30, 2009 — At company level, the main form of pay differentiation is through the proliferation of **ad personam guarantee** clauses, by which better economic ...

page 119
https://books.google.com.jm/books?id=mcl-DwAAQBAJ&pg=PA119&l...
**ad personam guarantee:** it is a guarantee that originates in the moment the punitive right is exercised by a judicial authoritiy, "exhausting" prosecution powers in the entire system. The outcome is a subjective situation consisting of the right for the individual of not being involved, any longer, in proceedings on the same facts.

In summary, I would leave "ad personam" (from Latin) guarantee", italicizing "ad personam". While a previous ProZ entry (which was Italian to English) anglacised the term, i.e. personalized/personal basis, I wouldn't. I would stick with the Latin form.