Like liquid-applied membrane systems, sealants are applied as liquids and then dry in place. They are successful if they are sufficiently flexible to remain in complete contact with the surfaces they connect as those surfaces expand and contract over time. Thus, sealants provide both long-term flexibility and adhesion.
Sealants are not used to create an airtight membrane on an entire surface of the substrate material (usually a panel of some kind). There are liquid-applied membrane systems for that purpose. Rather, they fill gaps and cracks in and between materials that are already sufficiently airtight.
In this regard, sealants are similar to tapes in that they bridge gaps and cracks in the airtight layer created by other materials. Even if a panel-type airtight layer is installed perfectly, there are always seams between panels. Sealants by themselves or in concert with gaskets and/or tapes will prevent air from passing through unavoidable seams.
Sealants are useful when there are accidental holes in a single panel. For example, a crack or nail hole in a sheet of plywood or OSB.
These sealants are often called “caulking.” However, sealants are not glues. Sealants are not intended to bind two or more materials together; they are adhesive only in order to remain in contact with the materials they join to prevent air movement between them.