Governing the use of such products is an American National Standards Institute guideline - specifically, ANSI A-118.10-93. This standard applies to thinset ceramic tile and dimensional stone installations. It covers trowel- and liquid-applied products and sheet membranes. The standard's main purpose is to develop materials that prevent moisture in wet areas from migrating to surrounding areas (or areas on a lower level) and thereby damaging them.
As I mentioned, currently available are liquid-applied membranes, sheet membranes, liquid mortar membranes, as well as membranes that utilize a mesh that is covered and embedded by one or more coats of a liquid-applied product.