A PBV was created and an attribute value was created with a List String and value was added during the creation of the PBV. Later a user of UCMDB changes the value in the attribute to another value from the list.
When a PBV is created an enrichment with the original value is also created. This enrichment has all the values for each attribute that was set during the creation of the PBV.
When a string type is used for the attribute value you can leave this blank but if a string list type is used for the attribute there is no way to leave this blank during the creation of the PBV.
One should consider using just a string type and leaving it blank if you are planning on changing this value manually after the creation of the PBV