In the Google Analytics help page https://support.google.com/analytics/answer/2649554?hl=en
Consider your long-term reporting goals and how they might affect your account structure. Create an implementation plan before you set up new properties. In some cases, you may want to have multiple resources all send data to the same property. In other cases, you may want each resource to send data to a separate property. In either case, you can use views and filters to create specific perspectives of your data.
My question is, when should I use separate property and when I should not? What are the cases?