When you create your party, each class will have a blurb about its defining skills and primary role, as well as two stats that have a minimum of 8 to set up its baseline. Since stats determine equipment proficiency, how you set those up is significant in how the character's role ends up playing out. For example, in my last playthrough I gave Moon high Endurance and Riposte and put her up front as a caster-tank, and that worked out decently. One of my playtesters used the default party, but ended up turning Simon (Life) into a ranged attacker and letting Alis (Sun) do most of the healing. Magic (e.g. Destruction) scales more with level than equipment, so the intent is for "caster" classes to be able to choose more balanced stats / equipment while still having reasonable skill damage, or to focus completely on casting stats and equipment to maximize skill output.
In general, the classes most suited to be put in the front row are War, Vengeance, and (slightly less so) Hunt, since several of their skills depend on weapon range. Vengeance's Overpower is strictly short-range, and War's Champion skill works best for a front-row tank (though it's still useful as a self-heal either way). War and Vengeance I believe are also the only two that start with 8 END minimum, so it's easy to boost them up to 12 for heavy armor. Of course, you can still put any of these three in the back with a bow or glaive. The other classes' skill sets are more magic-focused, so they can potentially do well in a hybrid front-row or focused caster back-row role. I ended up skewing the class distribution too far towards casters, I know.
It's not a perfect ruleset, so I'm sure there are gaps and things I didn't consider. I can't promise that literally any party build is viable, but as I mentioned I was able to complete the game without any of the primary healing classes, so that goes a long way to proving out build variance.