An excellent question. In effect, a Major faction is a faction that has a sphere of influence on our
Galaxy Map. Major factions are also the only factions to role-play large fleets, armies, and other military forces. Major factions can also participate in the
Warring Factions feature of Star Wars RP.net.
Minor factions do not participate in the above. They have no sphere of influence. They have no powerful fleet or army. And, they do not instigate Dominions or Invasions. The main sign of a Minor faction is when the faction does not have a sphere of influence.
Since the requirement for being both a Minor and Major faction is both to have 5 active, unique writers, being a Minor faction is by choice.