Dropdown Help: Private Attach Only
If you mark an object as "Private Attach Only", only page editors can add Children and Related objects. This is ideal for TRs, some articles, etc. Most of the time, you don't really want someone else uploading their own photos to your TR. The option is not available on Mountain/Route/Area pages (and a few others) since we don't want someone disallowing attachments to those.
Note that marking an object "Private Attach Only" only applies to children and related objects. That is, the object can still be made a child of another object. So, if I mark my TR as "Private Attach Only", someone else could still attach my TR as a child to their mountain page.