Originally Posted by whk
Can someone explain this feature or point me to where I can learn more about it?

I believe he may be talking about the image attachments, which are managed by the Attachment Manager.

The very last line of the "UBB.threads (2018-01-11) Changelog" section at:

"[NEW] Attachment Display is now customizable between Grouped or Inlined. Grouped images are displayed as thumbnails grouped together under the post. Inlined images are displayed at full width (while still fitting within the post width) just under the post text."

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