Hi @Shruti1 ,
When you decide to create an enumeration within the schema definition, use enum="abc" in attribute definition. OOTB Example, enum="gender" in nms:recipient schema.
When you decide to create an open or closed type enumeration in /Administration/Platform/Enumerations/ folder , use userEnum="abc" in attribute definition. OOTB Example, userEnum="city" in nms:recipient schema.
dbEnum are mostly used in OOTB schemas where those enumerations are 'System' type created under /Administration/Platform/Enumerations/ folder. Example, dbEnum="operationNature" in nms:operation schema and dbEnum="eventType" in nms:delivery schema.