The FVA simulation returns the predicted class of every reaction and compound in the model during growth given the input parameters. Reactions in the model are classified as forward essential or reverse essential if they are required for growth to occur, with forward and reverse referring to the direction in which the reactions must proceed. Reactions that are not essential for growth, but still active, are classified as forward variable, reverse variable, and variable, with forward and reverse indicating when reactions proceed only in a single direction. Reactions are classified as blocked if they cannot carry flux under the conditions specified by the user. Metabolites in the model are classified as essential nutrients or essential products if their uptake or secretion is required for growth in the input conditions, and they are classified as transported if they can be taken up or secreted but are not essential for growth. In addition to classifying the reactions and compounds in the model, the FVA simulation returns the maximum and minimum values for the flux through each reactions and the uptake/secretion of each metabolite.