It's located within the Attachments setting page of the Control Panel. It restores the post attachment presentation of the 7.5 series, while allowing you to toggle to the 7.6 attachment presentation if you'd like that instead.
Grouped attachment display is good for when the majority of your forum posts are technical or include many attached images at once, such as events.
Inlined attachment display is useful for when your users often post just a few (or only one) images at a time with each post.
The display type should be specific to how you'd prefer your post attachments to display.
"Grouped" is what is currently used on UBBCentral.