Lexeme forms and allomorphs may be constrained by their phonological environment. Parsers use environments.
You insert and edit environments in Environments (Grammar). This is where they are stored. However, you may type or construct environments directly in Environments fields (Lexicon Edit). See: Environments field (Lexeme Form) or Environments field (Allomorphs).
The Environment Help topics are the following:
Environment String Representation examples
Environments may use either natural classes and/or graphemes (phoneme).
Program-proposed analyses based on previous user analyses do not use environments.
For a discussion about environments, point to Resources on the Help menu, and then click Introduction to Parsing. See to 3 Morphophonemics.