Superseded. This 2015 example teaches a technique Ext JS 6 made unnecessary: the shared slot had to be declared as an object so a descendant could write through the inherited reference, because
set()did not yet climb to the ViewModel that declares a key. It does now, and a plain key works. The example and its demo are kept here for comparison — the current one is ViewModel Hierarchy · ExtJS, which measures both frameworks side by side and lets you cause each way of getting it wrong.
This example illustrates configuration of hierarchical view models and it is accompanying the blog post Configuring ViewModel Hierarchy. If you haven't read this post do it now or the example might not make any sense to you.