Hierarchical Models and non Hierarchical Models

what is the difference between Hierarchical Models and non Hierarchical Models in context of SQA?I have read his differenct quality models like dromeys,SATC and qmood quality models which all are Hierarchical Models.But when I started reading non Hierarchical Models,first question arise in my mind is what is the main difference between two?I could not find anything which satisfied my answer.