The Volume Performance Advisor will assume that all currently unassigned volumes are not in use, and may recommend the reuse of these volumes. If any of these unassigned volumes are in use, for example as replication targets or other DR purposes, and these volumes are recommended for reuse, the result could be potential data loss.
Go to the "Specify Settings" of the wizard and unselect "Consider volumes that have already been allocated but not assigned to hosts in the performance recommendation" if you do not wish to consider volumes which may potentially be used for other purposes. If you wish to continue to consider unassigned volumes in your recommendations, continue with the wizard.