As HansVader said, for PO it's a requirement to wear a pack. SWAT, you could choose a trooper without a pack to get approval. Head over to the Sandtrooper armour section and have a look at the pinned pauldron colour topics, specifically orange pauldron, so you can see some backpackless loadouts. If you build your pack right for the PO approval you can use it for SWAT approval though and it does open up your choices