Souls and their utility are extremely nebulous within Deltarune's setting, especially with Kris's being a stand-in for the player's agency therein- I wanted to leave that sort of stuff open ended within the ruleset as a toy for gamemasters to play with, or tables to interpret for their own games as opposed to setting down hard rules on what a top or charm soul would provide as a benefit. It speaks more to your personal preferences and tastes/character, you know?
Moreover while they're included in character generation, it's wholly up to the player/gm as to the nature of that soul. For example, the girl Monique in the ruleset's example party would be a FIGHTer class wise, yet is portrayed with what's essentially a weird player soul- Caroline on the other hand has a non-player soul, yet is the ACTor of the party and agreed leader of the bunch. If players/gm's/tables decide they'd like to elaborate on the different flavors of soul and the natures thereof for their games, I'd highly encourage it; Dialtone as a system is purposefully left open ended so that isn't hampered by strict rulesets.