What limitations exist for Food for Special Medical Purposes allowances?

The Food Standards Code was recently changed to permit the use of an amino acid in foods for special medical purposes. This amendment was made to remove a negative effect on trade arising as a lack of permission to include certain substances. The Standard dealing with Foods for Special Medical Purposes was designed to include broad compositional permissions to prevent this sort of scenario, so why was an amendment to the Food Standards Code required in this case? Under what circumstances will substances for use in Foods for Special Medical Purposes require regulatory pre-approval?