The Excel formula `I_START_MODE` prepares the model to run at specific task. This may include enabling insertion of additional seeds and memorizing the current values to register pending changes later. Parameters: - `mode`: External feature mode. Possible values for external feature mode are: - `None`: off - `AtRisk`: for use with Pallisade @Risk. - `AlterWorkbook`: for altering the workbook. Returns: no return value.