The Natural Classes Help topics are the following:
Natural classes are used by Environments (examples), Phonological Rules, Affix Process Rules (for the lexeme form and any allomorphs), and pattern-matching entries for GuessRoots in the Hermit Crab parser.
If you use one of the computational parsers, then natural classes are part of how the parser determines where allomorphs can appear.
You can insert natural classes that are defined by phonemes or by phonological features. Some common ones are consonants, vowels, voiceless stops, and back vowels.
Language Explorer may automatically create a natural class. See: Edit a natural class.
Create a Pattern-Matching Entry (using natural classes)