The use of dit with “an identifier” was a common practice in France at one time. It’s use helped identity an individual and distinguished them from other individuals in the community who had the same or similar names. “Dit” is French, meaning “known as” and Briard is French meaning “from the Region of Brie”.
Quite often, but not always, the dit name referred to the geographic or political area. In the case of Pierre LeJeune dit Briard, it meant that he was of the LeJeune family who resided in the Brie region, a region in northern France.
Brie is a very fertile farming region located about an hour’s drive east of Paris, France, near the Champagne Region. The city of Meaux is a major commercial center in the Region with a population of approximately 70,000.