Required Expression (formally Predicate Rule) Documentation

[Busthead]

I've search the Documentation but their doesn't appear to be a section describing the functionality of a Required Expression or Predicate Rule. Where is the functionality of this feature described?

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New App Features in 2.2.8

Release 2.2.8 introduces a number of new features, new apps, and new app features. These include Hub Variables, a new version of Rule Machine, Rule 5.0, Button Controller 5.0, and Button Rule 5.0. Hub Variables We have added Hub Variables as a Setting for the hub. Hub Variables are quite similar to Global Variables in Rule Machine, but implemented in the hub rather than in the RM parent app. This provides for the use of Hub Variables in other apps, not just in RM. Hub Variables are very eff…