Or 'vehicle subject to withdrawal of registration papers', 'vehicle suspected unroadworthy', etc.

I suspect this concerns the retrait conservatoire de la carte grise, i.e. pending expert appraisal and repair/destruction when the police consider the vehicle may no longer be roadworthy. See:

http://www.avocat-service.fr/le-retrait-conservatoire-de-car...

Since such vehicles are systématiquement expertisés dès leur arrivée (in order to determine if they are economically repairable or just good for parts/scrap), this would seem to fit.

"la mention “à titre conservatoire” par définition implique une mesure d'attente en vue de la prise de décision prochaine"
https://www.legalstart.fr/fiches-pratiques/gerer-ses-salarie...

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Note added at 42 mins (2023-12-19 15:49:01 GMT)
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And since the text says these vehicles are systématiquement expertisés dès leur arrivée, my first suggestion (title) takes us round in circles (well, it's not going to take us round in triangles or square in circles, is it now), so I would ideally reprioritize my proposals.


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Note added at 42 mins (2023-12-19 15:49:41 GMT)
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Pardon my shouting.


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Note added at 43 mins (2023-12-19 15:50:05 GMT)
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Pardon my shouting (bis).