Strength: Strong, when the witness has something to lose
What it proves
That somebody else observed the work being made.
What it does not
A witness with no stake is easy to dismiss. A cosigning editor, client or co-author is different.
How to get it
Ask the person who saw the drafts, took the calls, or reviewed the outline to say so in writing, on their own letterhead if possible.
How long it survives
As long as they will stand behind it.
Why it matters
One credible third party changes the shape of a dispute more than any document, because they cannot be accused of having a stake in the outcome - unless they do, which is why the stake matters.
Check the evidence in your own document
The analyzer reads the process record inside a .docx and reports each signal separately, with its limits stated.