The terms layer and horizon are sometimes used interchangeably, even by pedologists, but this is not technically correct. The difference in the terms meanings is rather simple and is discussed briefly in the Soil Survey Manual (p. 60). It probably should be explained more clearly in Keys to Soil Taxonomy. A soil horizon has properties resulting from pedogenic processes. A soil layer has properties inherited from the parent material with presumably little or no pedogenic development. Note that in the headings for the various master horizons and layers listed in the Keys to Soil Taxonomy, A, E, and B, only use the word horizon because they are always pedogenic. The headings for R, M, and W only use the word layer because they are never pedogenic. The headings for O, L, and C use both terms because they can be either.
Master horizon M – note that the key feature is that it results from human manufacture in an industrial sense. It is meant for things like covered asphalt, or geotextile liners and so the use of M only applies to anthropogenic soils. Suffixes d, m, and x are not meant for use with M.
The suffixes a, e, and i are considered connotative for sapric, hemic, and fibric organic materials respectively. The suffixes are the second letter of each word. Whether this was the best choice can be debated. However, changing suffixes after they are in use is problematic because it will result in confusion to anyone reading a soil profile description containing these suffixes. They would have to know which definition was used when the description was written.
Suffixes o and s are used to describe different soil processes. Suffix s is used to denote illuvial accumulations of sesquioxide in complex with organic matter. This is common in Spodosols. Suffix o is used to denote a residual accumulation of sesquioxides (not illuvial, but formed mostly in place by mineral weathering). This is common in Oxisols. While they both indicate the presence of sesquioxides, the pedogenic process is quite different for each. Consider, a Bo horizon in Brazil is quite different from a Bs horizon in Norway so both suffixes are needed.
Hopefully this adds some clarification to these questions. Perhaps the information in the Keys to Soil Taxonomy could be clearer in explaining some of these points.
