This indicates the extent to which the notation is understandable (as opposed to writable) by someone without formal training.
One of the perceived problems with formally expressed specifications and their notations is that their cannot be easily understood by most people. Hence gaining understanding of the system may be quite difficult.
One solution to this problem may be to integrate both formal and informal descriptions of the system requirements.
This can be done by incorporating into the method a mechanism for cross-referencing to natural language requirements.