The engine configuration settings that are applied to one or more DB instances are contained in DB parameter groups. Only Multi-AZ DB clusters are covered by
Tag: "Error saving: This parameter group XXXXX cannot be modified because it is currently being applied to DB instance XXXX. (Service: AmazonRDS; Status Code: 400; Error Code: InvalidDBParameterGroupS
Modify the values of an Amazon RDS DB parameter group
Read the following articles for more information about Parameter Groups in RDS. Parameter Groups in RDS. Cannot modify the parameter group error in AWS RDS.