When the industry says "a major", it means one of exactly three companies.
Three groups dominate recorded music globally: Universal Music Group, Sony Music, and Warner Music Group. Everything else (thousands of companies) is, by definition, independent.
A major is not just a big label. It is a piece of global infrastructure. Each major owns its own distribution arm (it does not rent reach, it owns the pipe), runs an in-house publishing company so it can earn on side as well as , and carries the balance-sheet muscle to write large advances and absorb the losses on records that flop.
That scale buys two things money usually can't: a long-horizon view of master ownership (a major is comfortable owning recordings for decades, knowing catalogue compounds) and dominant market share that gives it leverage with platforms, retailers, and playlist editors. For an adviser, "is this a major?" is really the question "how much infrastructure and how long a time horizon is on the other side of the table?"