This document explains what you are seeing in sections labelled "Only Before" and "Only After".

In various places, re-frame-trace allows you to inspect values like app-db. Knowing the current value is useful, but you are sometimes more interested to know how a value has changed. The value might be X before the start of this epoch, and ended up as X'. So re-frame-trace will be showing you X' but you might woonder how X and X' are different. What got added or removed, and what was modified?

To show such differences, re-frame-trace chooses to do a calculation best explained by this pseudo code:

(let [[only-before only-after _] (clojure.data/diff X X')]
   ...)

Remember X is the value immediately before (this epoch). And X' is the value after (the epoch has completed).

By looking at the docs for clojure.data/diff, you'll see that it calculates how two values differ, and returns a triple of values. re-frame-trace captures and displays the first two elements of this triple as "only before" and "only after" respectively. The 3rd element is ignored because it's what hasn't changed, which isn't interesting.

So, to correctly interpret "Only Before" and "Only after", you'll need to spend a bit of time properly familiarising yourself with how clojure.data/diff works, but it will be a worthwhile investment of your time.