Hi Suresh,

A class group is an instance of the Data-Admin-DB-ClassGroup class and corresponds to tables in the PegaRULES database. Class groups cause the system to store instances corresponding to two or more concrete classes that share a common key format in a single database table, which is known as a work pool. The name of the class group is a prefix of the names of its member classes.

You can refer below link for detailed description:-

https://community.pega.com/sites/default/files/help_v722/procomhelpmain.htm

Default value and list:-

If my understanding isn't wrong you mean to ask default value and list in generic sense, it refers to the pre-populated or value assigned to a property in absence of any value.

Correct me, if I sounds wrong.

You can populate default value, by either adding them as default value in property definition or else using data transform by pre-populating these values and calling this data transform as pre-processing.

For populating default list, 

It is populated based on index referred, though if you want to populate a value for particular index, then you should use "Property-set" method in an activity.

Kindly acknowledge, if it helps.