Canning Headspace Guidelines
Headspace is the space from the top of the jar to the food or liquid in the jar.
Correct Headspace Guidelines -- As a general rule:
Why is it important?
Too little headspace -- Some foods, especially starchy foods, swell more than
others and require additional headspace. If too little headspace is allowed, as
food boils inside the jar it may be forced under the lid, leaving residue on
the sealing surface and possibly preventing the lid from sealing.