According to Michal Frankl from the Department of Modern Social and Cultural History at the Academy’s Masaryk Institute and Archives:
"Among the dozens of incidents marked in the map is the case of Arnošt Polák, who was arrested for failing to mark his Identification card with the letter J. For that he faced a fine of 5,000 crowns or five days in prison. I guess that was one of the very common incidents that happened in Prague in those days...
“While we were working on the project we combed through the files of the Prague police and we came across approximately 17,000 such incidents.
“This can include cases when people were arrested for some kind of special trespassing, or acting against anti-Jewish decrees, such as that they were outside without the Star of David attached to their clothes, that they entered a park, they were in a shop outside of the very limited hours for Jews, or for instance they wanted to travel on public transport in ways that were not allowed to them."
You can read more at: https://english.radio.cz/new-app-tracks-holocaust-it-happened-prague-streets-8740371.