Synthetic keys. When two or more data tables have two or more fields in common, this suggests a composite key relationship. Qlik Sense handles this by creating synthetic keys automatically. These keys are anonymous fields that represent all occurring combinations of the composite key.
Common Fields between 2 or more tables it is combination of same columns.
Circular Loops : Qlikview & QlikSense works on associative technology , for this we need data path in data model 1 : 1
Circularloop means you have alternative paths between two tables, so you need at least three tables in your data model to run into this. QV can't live with circularloops, it will break the loop by loosely couple one of the tables automatically.