When I was issuing paper tickets manually many years ago, which I did for my first 3 years in the airline industry, apart from the 3-digit IATA accounting code at the beginning identifying the issuing airline, and the final check digit, if memory correct the first digit after the accounting code matched the number of flight coupons in the ticket form being used (1, 2 or 4 at the airline I worked for), and the remainng 8 digits were just sequential serial numbers as on any other type of ticket or document requiring a unique number.
In your example above, the first three digits of the document number (211) indicate the type of form. For example, a trip pass, an MCO, an automated ticket, and a PTA would all have their own unique, three digit form number. By looking at the first three digits of a document number, an agent could identify the type of document. The remainder of the number (7 777 256) is the serial number.