Let me try to answer this part by part:
What is the use of creating a Class - with options Concrete and doesnot belong to classgroup . - This is for creating data classes
If it doesnot belong to classgroup then it is not mapped to any Data Table in DB and cases are not persisted as it is not mapped to any DB table - Please note that data tables and DB tables are not the same thing. data tables are just data classes that have a UI. DB tables are physical tables in the database.
If this is true then what is the significance of having these options while creating class- Imagine that you go to a library. You need to borrow a book. For this transaction, your name, the ISBN number of the book and the dates of borrow and return will be enough. So that's the data that pertain to your 'work'. The author of the book , the name of the publisher, or your full address, the date since when you are a member in the library are not necessary for the transaction to take place. However, these are necessary if you want details of the books in the library, or the members of the library. These type of data are related to data classes. And the data classes can be mapped to a table of your choice too.