Organize your Built-on Applications such that the Ruleset stack for any Client Application positions all Pega-provided Rulesets (including those from Pega's applications) at the bottom of the Ruleset stack, and Client Rulesets above them. Pega Applications are not tested in scenarios that inject other non-Pega Rulesets in between Platform and one of its Applications.

With this approach, any Class introduced in a Client Ruleset can direct-inherit from a Class provided by Pega Platform or a Pega Application. New or improved capabilities added to those Pega Classes via upgrades become available to your Client-layer subclasses through Inheritance.

Similarly, consider defining distinct Built-on Applications for Client Enterprise and Reuse (formerly, "Framework") layers so that their Rulesets are beneath those from your Case layer. Classes in your Case layer can follow direct- (or pattern-) inheritance paths through more generic Client Rulesets, and then direct-inheritance paths to Classes from Pega Rulesets.

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.