http://www.codices.coe.int/NXT/gateway.dll/CODICES/laws/fra/...$f=templates$3.0
Contexte légèrement différent mais semble décrire le même genre de situation:
La Cour constitutionnelle peut accorder le statut de partie subsidiaire à la procédure aux personnes ayant démontré leur intérêt légitime concernant l'issue de la procédure.

An “Additional Interest” is a party listed [i.e. just listed - not signing/agreeing to anything => "tiers"] in an insurance policy that has an “interest” in being notified whenever a policy cancels or has a major change made to it. In other words, this party is simply being made aware of the change – nothing else. There is absolutely no coverage whatsoever afforded to an Additional Interest.[i.e. an additional interest is not party to the contract//it's an "outsider" with only the right to be kept informed] An example of a party who may need to be listed as an “Additional Interest” is a loan servicing company who is managing the loan for a seller-financed transaction." [as the loan servicing company has all the rights to claim the insurance payment for itself, if there are outstanding amounts on the loan]

"tiers intéressé" is someone who's not party to the contract, but has an interest in the contract.

as in:

"... dans le but de ne pas verser d'indemnité à l'assuré ou au tiers intéressé par le contrat ..."

http://www.memoireonline.com/12/10/4173/m_La-reception-du-de...