This is merely one section of many on Ultralight Backpacking. Refer to this post for other categories.
There are a ton of cottage and specialty UL backpack makers out there and this rabbit hole is particularly large and inviting. For UB, you only need a 30L or 40L size. L is for litres and is a unit for the volume of the pack. Anything over 40L is really a specialty backpack for extended wilderness trips with no resupply points. Close to 2 weeks and more. Unfortunately, most of the major backpack manufacturers don't make UL backpacks. These are all under 2 lbs (910g) for 40L and under. For 40L and above you are looking at 2.5 lbs (1100g).
It's important to remember that these packs, even the large ones, are meant to carry less weight. I had a friend in University that would often go on 2-week desert adventures with a 70 lb (32kg) pack! Not only is that old school, but it’s unnecessary self-inflicted punishment. Nowadays that same adventure would be a 30-40 lb (18kg) adventure. These packs are not designed to be loaded super heavy and often max out at a 30lb weight limit, even for the larger ones.
Osprey has been making the most comfortable packs with the best ventilation for over 20 years. Their typical packs however are very heavy at around 4 pounds and up. They are new to the UL pack scene and have done well with the Levity 45L below. This pack is 1 lb 13 oz (839g), their lightest and has a load capacity of 5-20 lbs. Outdoor Gear Lab has tested it with 30 lbs with good results. This pack is one of the most comfortable UL packs with exceptional ventilation. Most of the UL packs have minimal to no ventilation in the pursuit of shaving grams. The Levity is made with very lightweight nylon and polyethylene which make it extremely lightweight but, perhaps less durable than some of its counterparts. There are others out there made from heaver materials but keep in mind that these options, although more durable, are heavier.
Some use them for saving the legs on trails with a lot of vertical gain while others use them for added stability on any trail. In either case, for UB it’s important to consider that some UL shelters are designed to use hiking poles to set them up, thus combining function. If you haven’t used hiking poles in the past, this may be reason enough to give them a try. The Big Agnes Tarp set up at the beginning of this post shows what this type of set up looks like. What ever hiking pole you do decide on, try to get either the folding kind or the telescoping kind with the clamps. If choosing telescopic, make sure that the clamps have a little set screw to adjust the tension on the clamp if it gets loose. The telescoping kind that twist to extend tend to wear out sooner, don't stay at the length you'd like and are hard to fix in the field. Some poles are even fixed length and made of carbon for the lightest option out there and nothing to fiddle with. The Black Diamond Alpine FLZ below is sturdy but heavier than some of Black Diamonds other options. This style is nice because they fold down so small. The Distance Carbon FLZ Trekking/ Running Poles are a similar Z shape when folded and very light. They are more delicate and not as sturdy but are very lightweight. The telescoping kind are sturdy and easier to extend and collapse than the Z type, but they don't fold down as small.