What classification difference exists regarding the prominence of the Perseus Arm?

Answer

Some sources refer to it as one of the four major arms, while others classify it as a minor or secondary spiral arm

The structural classification of the Perseus Arm varies, with some sources naming it one of the four major arms, while others view it as a minor or secondary spiral arm less massive than giants like the Sagittarius Arm.

What classification difference exists regarding the prominence of the Perseus Arm?
galaxyastronomyDistanceEarthMilky WayPerseus Arm