Every local data storage is associated with a class in Pega. It's a storage feature which allows the user to store data records for a data type within Pega application without a need for manually creating or maintaining database tables.
To add record using an activity,
1.Navigate <YourDataType> -> Records(Tab) ->Actions-> Show Database Information.
2.Now you can perform all "Obj" methods on this class to add, delete or update records as per your business requirements through an activity.
Hope it answers your question, kindly notify if it does.