Not really the same thing and has to do with an insufficient number of seats in a particular fare class bucket to book all passengers in that fare class on same resevation (in which case, they will all be bumped up to a higher class fare in which there is sufficient inventory). The fare filing itself is not preventing you from booking multiple passengers in same fare. The Delta website will actually tell you when inventory in a particular class gets down to 5 seats or less. In the case below, there is only 1 seat left in the FC "I" class and the cheapest applicable I class fare goes for $769. The maximum number you can book on one reservation is 9 passengers which is also the maximum number of seats in fare inventory buckets on flights.






Attempting to book 2 pax on same reservation will bump them both up to a $909 D class fare (there are 4 seats available in D inventory class on flight). DL FC/D1 fare classes are J/C/D/I/Z.