The ContentApp page objects can select "nodes" on any content app by a given "path" via
#selectRowByPath(path). 
(Please read the javadoc of the method to properly understand what the path parameter means). That works fine and as expected - even on the Definitions app.

definitionApp.selectRowByPath("/apps/about");
//works

On the JCR browser - #selectRowByPath also works for a path to an attribute:

jcrBrowserApp.selectRowByPath("/ftl-sample-site/main/0/text")
//works

On the Definitions app, selecting an "attribute" fails. (Well, the row looks selected, but ut us not green but grey, and the actions are not available)

definitionApp.selectRowByPath("/apps/about/appClass");
// fails