Well, you can see the class libraries as the stuff that you think would be nice to have in general beside the classes etc. provided my the framework. So it is really a matter of abstraction.
Class libraries are to be designed generic so for the database example there should not be any information about a specific database, table etc. Perhaps you would like a method to extract a column from a specific table. The signature might look as follows...
public somedatatype ExtractColumn(string connectionstring, string table, string column)
... and somedatatype
is the format in which you in general would like to get you extract. This is a simple example but keep in mind that the methods have to be generic to be reuseable.