Next, an mzSpecLib library can list any number of "<Cluster=${key}>" entries, describing related groups of spectra inside or outside the library, followed by any number of "<Spectrum=${key}>" entries, describing library spectra including any acquisition or generation attributes, interpretations as sets of zero or more analytes, and a peak list that may include peak annotation or aggregation statistics. All of these entities are defined either in terms of controlled vocabulary attributes or via an externally defined grammardata model pair permitted by the specification.