Some vacuum cleaners are not only designed to clean your home but are also made to deliver a healthier living environment by capturing and preventing the contents it sucks up from leaking through. If a vacuum cleaner performs this function than chances are it features a sealed system. This goes hand-in-hand with true HEPA filters as with the both of them, you are guaranteed a vacuum that will substantially improve the living environment of your home.
Basically, without a sealed system there is a chance that the dirt and debris can leak through the vacuum after it has entered inside. While you may need to spend some extra money on the models that offer this system, it is more than worth it if you struggle with allergies and generally feel the living environment of your home is unsanitary.