1- The M ( master horizon ) is used for manufactured liners such as plastic, rubber, geotextile fabric, concrete, or asphalt used to intentionally seal a soil from water and root penetration. An example is the geotextile liners used in landfills, or thick plastic used to line a retention pond. The u ( suffix) is used to indicate the presence of artifacts (defined in Chapter 3 of the 12th Ed. of the Keys to Soil Taxonomy). It is not used for incidental litter that may be plowed into the surface of farm fields, and is not used in floodplain deposits where litter in streams becomes buried. There must be an intent by humans to add the artifacts, or they must prove the construction of an artificial landform. An example would be pieces of plastic and brick in the profile of an artificial landform in a city.
2 - Yes, you can apply the u suffix with B master horizon. Identify the master horizon based on its properties relative to other horizons in the profile, then see if any artifacts occur. The u can be used with any other lowercase letter (suffix).
3 - The M horizon is a layer made of a manufactured material, so there is little reason to add the u. Concrete contains artifacts, and after it has weathered in a humid environment, you could identify the artifacts left behind.
