What is a unique design feature of the 20-man life raft?

The unique design feature of the 20-man life raft being always right-side-up after inflation is critical for ensuring the safety and effectiveness of the raft in emergency situations. This feature significantly enhances the usability of the raft in life-threatening scenarios, as occupants can enter it without the risk of it overturning. The design takes into account stability and ease of access, allowing those in distress to quickly and safely board the raft without having to flip or adjust it after inflation.

In contrast, while features such as compact storage, rapid inflation, and the inclusion of survival supplies are also important aspects of a life raft's functionality, they do not uniquely define the 20-man life raft in the same manner as its ability to maintain an upright position post-inflation does. Each of those features contribute to the raft's overall effectiveness and safety but do not stand out as the defining characteristic that ensures immediate usability.