An example is discussed in Implementing specializations of Pega OTB Classes. 

The primary benefit for layer-specific subclasses is the flexibility it provides in allowing the overriding of capability from / adding new capability to Pega-provided classes whilst allowing your client's applications to continue to use the Pega class as intended when appropriate.

In the example referenced above: 

• Ruleset-specialization of PegaData-Address.PostalCode into a client layer forever masks the Pega-provided capability of that rule for any application where that Ruleset is on the Ruleset stack.
• The same specialization of PostalCode - implemented as a Class-specialization to MyOrg-Data-Address - still allows client applications to opt to use the Pega-provided capability by referencing PegaData-Address in their data model.

Discussion on this topic was sought from the LSA Data Excellence (Pega 8.4) webinar conducted in July 2020. The webinar and its full set of discussions that arose from it are available at LSA Data Excellence: Webinar, Questions & Answers.