A commercial backpack vacuum is a vacuum cleaner that's worn like a backpack and commonly used by cleaning companies. You can also find backpack vacuums made for residential use that cost less and don't have as many features. Commercial grade models are designed to be used heavily every day such as by cleaning companies or in industrial applications. Because of that heavy use they have to be constructed in a way that will allow them to stand up to the abuse.
If you want a backpack vacuum for your home, you're probably not going to want to spend the hundreds of dollars, and sometimes more than $1000, for a commercial grade backpack model. If you're planning to buy one of these to use in a professional cleaning or commercial application, then you won't want to buy a residential model that can't stand up to the heavy daily use.
A very important consideration when purchasing a commercial backpack model is the weight and how well it fits. The straps should be heavily adjustable to fit people of all body types. And the backpack itself should rest on the back so that the weight is on the hips and shoulders and not on the lower back. Commercial models are typically worn for a long time because there's a much larger area to be vacuumed, so comfort is very important. There are models that are designed like fanny packs that only sit on the back of the hips, which may be best for people who have shoulder or upper back problems.
Most companies that purchase a commercial vacuum of this type get those that have at least a two-filter system. Many prefer up to five filters for the best air-quality coming back out of the machine. If you're investing in this type of vacuum, get one with as many filters as you can to keep dust and debris from being blown back out into the room that's being cleaned.