The Zpacks Arc Haul Ultra 50L is a women's-specific thru-hiking backpack weighing 586 g, working out to about 11.7 g per litre. Like the rest of the Arc Haul Ultra range it's built from Ultra 100X fabric and carried on a carbon-fibre Arc Frame, an external arc that lifts the back panel away from your spine for airflow while a padded hip belt takes the load off your shoulders. Back length adjusts within the women's-specific fit rather than being fixed to one size.
At 50 litres this is the second-smallest option in a range that runs from 40 to 70 litres, a practical size for standard multi-day thru-hiking resupply intervals without carrying the extra bulk the 70L allows for bear canisters or heavy cold-weather kit. It suits hikers already running an ultralight-style base weight who want Dyneema durability and real frame support rather than a frameless pack. If your loadout regularly needs more room, the 70L is the better fit; if you're packing tighter, the 40L saves further weight.