You can have up to eight distinct reference modes in a model. These can be managed from the "Ref Modes" tab of the Model Settings dialog:
Click on the reference mode you want to change.
Delete Selected will delete the selected reference mode from the model. The dataset displaying that reference mode will not be changed, but can be deleted from the Datasets tab of the Control Panel.
Rename Selected will rename the selected reference mode. The old name will still exist as a dataset, but can be deleted from the Dataset tab of the Control Panel.
Import .dat File allows you to take the information in a .dat format file and include it as a Reference Mode.
Versions of Vensim prior to 5.2 did not support Reference Modes. You will still be able to open models containing Reference Modes in earlier versions of Vensim but the reference mode information will be lost. This may happen even if you do not save the model in the earlier Vensim version.