Before you get to the field, you’ll create loadout’s for your different classes, made up of weapons you’ve earned, and accessories you’ve unlocked. Your progression, however, is generally horizontal – the more powerful an item is, the more “points” it requires from your loadout in order to keep PvP balanced.
With a few exceptions, each character generally has access to both ranged and melee attacks at any time. For example, if your assault unit has sword, axe, or power fist in one hand, they’re going to have a pistol in the other.
There are exceptions, of course. Heavy guns prevent you from having much melee capability, of course, and if you choose to equip a power fist and a storm shield then you’re not going to have any ranged options in your loadout. But your average soldier on the field is able to do both in some capacity.