"This file is also used by a post outside this batch"

Quick answer

The file you are about to overwrite is also what another post points at, and that post is not in this batch — so the replacement is refused rather than silently changing a page you did not select.

Only the affected rows are refused. The rest of the run goes ahead normally.

Why it is refused rather than handled

One file on disk can be referenced by any number of posts. Overwriting it changes every one of them at once. When all of those posts are rows in your batch, that is fine and the plugin handles it: each one gets its own copy, and every page that changes is a page you chose.

A post with no row in the batch is a different situation. You did not select it, you may not know it exists, and nobody would tell you it had changed. So the answer is to stop and say so.

Giving the selected post its own new copy is not offered either, tempting as it looks. That would move the file your post points at to a new URL — and you asked to replace a file, not to move it. Anything linking to the old address from outside your site would still be getting the old document.

What the message tells you

The refusal names the other post, its ID, and which field points at the file. That is enough to go and look at it and decide.

How to resolve it

• The other post should get the new file too. Add it to the batch and match it to the same file. Both rows will then be in the batch, each gets its own copy, and the run proceeds.
• The other post should keep the old file. Point one of the two posts at a different file first, then re-run. Once they no longer share, the refusal does not apply.
• The other post is stale. If nothing should be pointing at that file any more, clear the field on that post and run again.