What?
Just what the title says: A new service class between Economy and Business class that is rather common in the real world these days.
Why?
While it adds relatively little actual benefit, it makes things feel a tad closer to reality.
When?
For technical reason, this is probably best implemented when Custom Booking Classes are done. Also, because a lot of features (like service profiles and statistics) are currently hard-coded to three services classes + cargo, the effort required to implement this is actually quite high.