There are three different stucco systems in common use:
Typically, EIFS is a layer of foam laminated or mechanically fastened to a substrate with mesh and a polymer base coat over the top of the foam. It also includes an acrylic finish over the top of everything.
Conventional – or three-coat – stucco will have a mechanically fastened weather barrier or sprayed or rolled-on air and moisture barriers. Over the weather barrier will be expanded or woven wire lath, as well as a Portland cement scratch and brown coat. The finish can be a variety of products used individually — or in combination — to provide color and texture to the surface.
The weather barriers and lath for these systems are similar to three-coat stucco. Typically, a one-coat system has a layer of foam insulation (referred to as continuous insulation) between the lath and the Portland cement-based coat.
These are terms associated with one-coat and three-coat stucco systems. They are described as hard coat systems because the base coats used for these wall assemblies are formulated with Portland cement as the "backbone" of the assembly.
The strategy behind water barrier systems is never letting any water past the cladding. In other words, when built right, the water never penetrates past the outer surface of the wall to the inner cavity, where the moisture can affect the wall cavity. This is difficult but possible to do; however, once there is a failure, it can quickly become a problem because large volumes of water can be trapped for an extended time before someone knows it. This can result in rot, mold, and expensive repairs.
Water management systems make the assumption that water will eventually work its way past the outer surface of the wall. But when it does, a system of flashings, weather barriers, and metal or plastic screeds will direct water back to the outside of the structure. All Portland cement/hard coat systems — either one coat or three coats — are based on the "water management" strategy.