The Applies To class of Data Source is just like another rule.

Eg: we can have a data source "X" has a definition at PegaCA-Work level and have a different implementation at X-Y class level.

To explain more on info mentioned in the documenation, Pega Customer Service Application replies Data Source rule meta data while creating the cases (intents) from Case Designer to create the intent task rule associasted with this class. OOTB, when a case is created using in Case Designer from Designer Stuido, we would need to create case in some default Category which is evuantullay identified using the data source rule  called  "Primary". 

please let us know if this helps!

Could you explain why the Data source rule called "Primary" must apply to the class "Organization-ApplicationName-Work" while there are a couple of OOTB data source rules even not applies to a work class?

Usually the primary data source would applies to Interaction class (which inherits from org level work class). The reason for Primary data source to be applies to interaction class is that 

                   1. It represents the Interaction so that intent whens can utilize interaction level properties during evaluation.

2. Typically it is being identified by General Intent category and used while creating cases using case designer in general. but, it is not mandatory that default category should be identified by Primary data source. It can be any class in the system.

Yes, The data source rules need not be applies to work class always. It can applies to any class in the system. You can think of data source rule is partially like Data Page rule which is used to contact external or internal systems to bring the data onto clipboard. For example, you can have a data souce called Customer with applies to Org-Data-Customer which can be used to get Customer data from external system. 

Let me know if the above info answers your questions.