Because of the variant nature of syntactic annotation, and the almost infinite combinatorial possibilities, it is suggested that no part of the syntactic annotation be regarded as obligatory. The first layer (a) in the `hierarchy' of annotation, bracketing, could be seen to be a possibly obligatory level, and indeed for a constituent analysis, it would be. However, as we have seen, there are schemes in existence already that do not actually group together the words making up the constituents (ENGCG), and although this can produce a rather shallow analysis, it must still undoubtedly be regarded as a form of syntactic annotation.
As mentioned earlier, syntactic annotation may be undertaken for a variety of reasons, and therefore to impose an obligatory level of annotation would be to assume that there is a level of analysis common to all of these aims. This is not the case -- for some purposes (e.g. some kinds of parser testing), only bracketing, and no labelling may be necessary. For others the labelling of certain constituents, e.g. noun phrases, may be required, while the rest of the sentence may be left unannotated. For all these reasons, the obligatory level of annotation is to be left empty.