The standard Lead Rating field has picklist values of Hot, Warm, and Cold.

A restricted picklist is a type of picklist that enforces the integrity of the picklist values by allowing only values defined in the picklist during data entry or import operations. If the restricted picklist checkbox is checked for a picklist field, then any records with values not defined in the picklist will cause errors during import operations. However, if the restricted picklist checkbox is unchecked for a picklist field, then any records with values not defined in the picklist will be imported without errors. References: https://help.salesforce.com/s/articleView?id=sf.picklist_limitations.htm &type=5