Many soil classification systems are strictly hierarchical and have many categoric levels (like Soil Taxonomy or the German System), others have a looser hierarchy and more flexibility (like WRB or the French Référentiel Pédologique).

In most hierarchical systems each categoric level allows to add just one soil characteristic that may be expressed by one feature or by several features that typically occur in combination. If there are more categoric levels than characteristics worth to be mentioned, levels are filled up with words like Typic, Haplic, Orthic etc. If there are more important characteristics than levels, some important characteristics remain ignored. Moreover, a certain decision on one categoric level offers a specified bunch of possibilities for the lower-next level and ignores the possibilities other decisions would have offered (unless they are repeated). Contrary to that, systems with more flexibility allow to handle different amounts of characteristics and completely different characteristics at the same categoric level. Amplifications with poor information content (Typic, Haplic, Orthic etc.) are not necessary, and important information will not be lost.

For soil maps, the requirements are different. A strictly hierarchical system allows the mapper to take his mapping units from the first level or the second level etc. A flexible system leaves the mapper with the task to create his own mapping units by deciding himself which of the characteristics should appear on the map.

Soil Taxonomy has six categoric levels, four strictly hierarchical and two flexible ones. WRB has two categoric levels, one is strictly hierarchical and one is flexible. For classification purposes, the flexibility of WRB is more suitable than the rigid hierarchy of ST. For mapping purposes the stricter hierarchy of ST is better to handle than the fuzzy flexibility of WRB. We should see, how the two approaches could profit from each other.