From the RiveScript Working Draft:
Topics are able to include and inherit triggers that belong to a different topic. When a topic includes another topic, it means that the triggers in another topic are made available in the topic that did the inclusion (hereby called the “source topic”, which includes triggers from the “included topic”).
When a topic inherits another topic, it means that the entire collection of triggers of the source topic and any included topics, will have a higher matching priority than the inherited topics.
Basically, if you use “include”, the entire pool of replies in both the source topic and the included one are all treated equally as though they were all in the same topic. When you “inherit” a topic, then the source topic (and its included ones, i.e. its equals), all have a higher priority than the inherited ones.
So if you had a trigger of simply “*” in the source topic, then it would be impossible to match triggers in any inherited topics, because even though * is usually the lowest-priority trigger (which ends up dead last when the replies are sorted), it’s a higher priority because of the inherits option. If you used includes instead, it would be sorted dead last as normal.