okay, did a bit of research on acrylpro site and this is what I learned.. ANSI (American Standard Specifications for the Installation of Ceramic Tile) is an organization that sets-up performance standards for mortars designed to set ceramic tile. A mortar meeting A118.1 is an unmodified, dryset mortar that doesn't contain a polymer. Unmodified mortars are adequate for basic tile jobs such as a residential floor over concrete or a wall tile job when setting ceramic tile. For more demanding installations a polymer-modified mortar is required. Polymer-modified mortars meet ANSI A118.4 standards. You can use a mortar that is already polymer-modified (latex is in the powder) or an unmodified mortar and a liquid latex additive that is used in place of the water requirement. Installations requiring a polymer-modified mortar are when setting porcelain tile or large tile over 12” x 12”, in wet areas, high traffic areas, exterior installations, commercial installations, surfaces that could be subject to minor movement, installing tile over vinyl flooring, plastic laminates or other hard to bond surfaces or setting tile over a waterproof membrane. Installing tile directly over exterior grade plywood requires a mortar with special bonding capabilities, so these installations require a mortar that meets A118.11 standards. Also, the mastic although may seem to meet the requirements for ceiling tiling, isn't better than the polymer-modified mortar I am using.