The right of prior personal possession is a principle of intellectual property that allows a person to claim a right to an invention (patent) based on their knowledge/possession before a patent application (or utility model application) filed by a third party, without having to file any application themselves.
To assert this prior personal possession right to an invention, it is necessary to prove that the invention was in your possession before the third party filed the patent. There are various ways of proving this date, including the Soleau envelope (now e-Soleau), laboratory notebooks, and the filing of documents with a bailiff (now a judicial officer).
This right of personal possession is personal and cannot be transferred. It only applies to the territory or territories where the invention is possessed. Thus, personal possession obtained in France does not entitle the holder to exploit their invention in the rest of Europe.
In summary, this right allows an inventor who has not sought to protect their invention to continue to exploit it even if a third party files a patent application after the invention has been obtained. However, this right remains limited since it only applies to the territory where the invention was made and cannot be transferred.