||You're not missing anything Mathieu, except you do seem to be confusing WGS 84 with UTM. The former is a datum--a description of the size and shape of the earth used for georeferencing--while the latter is a coordinate system based on the TM projection.
The full UTM system includes a series of 60 partially overlapping meriodional strips, each subdivided by ranges of latitude designated by letters A, B, ... (they don't go all the way to Z; they stop at X). The use of letter designations has its place, but not (usually) in a GIS. For GIS work one doesn't need to know the redundant letter code for latitude. Instead, all that's needed is to know the zone and whether it's in the northern (N) or southern (S) hemisphere. There definitely is opportunity for confusion because "N" and "S" now have two distinct meanings each, so you have to know the context in which any UTM coordinate is named.
The Wikipedia article on UTM provides details. See especially the paragraph headed "note of caution." http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system