Not Unix or COBOL... it is the old IBM operating system that was specific to Airlines (later banks I think). The OS was ACP - Airline Control Program and then later TPF - Transaction Processing Facility that was designed for high volume processing. The applications (Pricing, ticketing, PNR maintenance, seat assignment etc) were originally written in IBM Assembler (ALC) and not COBOL. The PNR record allowed for flight segments that allowed a number of passengers and a booking code so all passengers in the PNR had to be booked in the same RBD (Reservation Booking Designator) though they could have different fares (for example different passenger types - ADT for the adult fare and CNN for the child, or ADT and CMP for a companion fare). So when you display the PNR you can see multiple passengers but only one itinerary.

A lot of the old limitations carried forward even with new technology and platforms.

For example - electronic tickets kept the paper ticket limit of four coupons (flight segments) per ticket. Anything with more than 4 segments still require multiple ticket numbers (conjunctive ticketing) even thought the old paper tickets are gone... (pretty sure that is still true)