There was a time in which an authorization was needed from one's home country to go abroad. Inhabitants of what was then the "soviet bloc" could not use their passport freely.
Travel was only allowed for short periods and under rigorous conditions, one of which was the time frame between the request and its eventual confirmation.
If the passport was granted (which was not frequent), only more twists and complications followed when trying to get a visa to travel to a specific destination.
Afterwards, from that destination one could only travel further through a new request at the embassy of each country. Those visas depended on questionnaires, time frames, and other complications of the Cold War. If the passport was not given to the "Office of Passports" of the country three days from the agreed date, it was then confiscated.
Opalka's response to the situation described above was the "Cartes de voyage". Initiated in 1972, they allowed him not to interrupt his work when travelling in a country belonging to the Warsaw Pact. He always finished the "Detail" in work long before a possible trip and continued to inscribe the continuation of numbers in black ink, on white ordinary paper, A4 format.