System seniority is what determines vacancy awards and relative seniority within base/equipment/seat (BES). You are given a system number on day one. Relative seniority determines your QOL. The more senior you are in your BES, the better trips you get, better vacations, better days off etc. Someone getting a NBCA award as a new hire would be super junior for years while senior pilots with lower seniority numbers bid on top of them. Most people don’t bid captain vacancies until they can do it at the seniority of their choosing, and every pilot already on the list when you show up will be forever be senior to you. Someone getting NBFO will gain relative seniority much quicker and enjoy a higher quality of life, although with a lower pay rate than a NBCA of the same seniority. NBFO is the lowest pay grade that we have and very few people will bid on top of you, especially on the 320/737 fleets. NBCA is much more senior and your relative seniority won’t move much for years due to others bidding over you.