Where did this value come from?

Find the call, the step and the Set Values element behind a value in the data, and see what each call changed.

A field holds a value you did not type, or a value is not what you expect. Somewhere a call, a Set Values element or a user action put it there. The panel keeps enough to point at the likely cause.

Ask the row

Open the Data tab and find the key: type part of its name in the filter box, or pick it from the tree.
Tick Track changes and click through the script, so the panel logs the changes. Without it, only the Set Values part below is known.
Right-click the row and choose Where is this set?. A small popover says what the panel knows. See Where is this set?.
Read it from the top: - the last change it logged: the time, the step, and the value it was before and became; - the call that was running, or had ended less than 5 seconds before, when the change was noticed; - the Set Values elements that write this key.
The call and each Set Values element are links: the call opens in the Calls tab and the element in Structure. Press Esc to close the popover.

Nothing is shown for what is not known. Hover a row that changed for a tooltip such as "Last changed 10:42:03 after Onboarding_GetPlans".

Go to the element that owns the key

Hover the row and press its Structure icon, or right-click and choose Show in Structure. The Structure tab opens with the element that owns that key selected. See Show in Structure.
On the Current value tab, Written by lists the Set Values elements that write into the element, each with its step. With Advanced on, the Visibility tab lists Written by too, with Depends on and Used by. See Depends on / Used by / Written by.
Open a Set Values element: its Set Values tab shows each assignment, the value it computes from the data now, and what the target holds. A row that differs is marked.

Read what a call changed

Open the Calls tab and select the call.
Open its Changes tab. It shows how the script's data changed because of the call: the last read before the call ended against the first read within five seconds after. If nothing was read in that time, the tab says so. See Changes.
If an element of the script makes the call, press Show in Structure in the detail head to see what it sends and where the answer lands. See Show in Structure.

When the answer is "not known"

If the value is (not in data), the element that should have written it has not run, or wrote another key. If the popover shows no call, none ran in the five seconds before the change, so look at the steps before it with the Timeline under Advanced. To find out why a call failed, see Why did this call fail, and what did it send?.