This Rules Annex, like the ones from previous editions, is filled to the brim with new ship system and optional rules. Also included are rules updates to the core rules themselves, the majority of which is centered about tweaking the construction rules for better balance (though nothing too wild as too completely invalidate older ship creations). This rules update has since been applied to the core book proper.
A good chunk of the book is dedicated to new movement system. Included are a very simplified inertialess system, a slightly simplified version of the default movement system, and a new vector-based movement system which is the most realistic of the bunch (or as realistic as you can get with 2D). Looks pretty fun, but can probably go a bit out of hand when there are too many ships around.
A fun new option introduced here are System Half-Hits, which result in a more even allocation of damage over a ship. Another huge new addition to the rules are the Customized Range Bands. Now every weapon can have its own custom range bands and a different weapon profile for each range band. This not only removes the need for the old range-based weapon traits, but also makes it much easier to accurately convert ship designs from other systems.
The one thing I didn't really like is the Point Defense System, which is pretty much the old Anti-Fighter Battery. I just prefer how Unity turned the AFB into a purely passive effect.
Another odd thing is the lack of new rules for Fighters (outside of mentioning how other new rules and system interact with them). Though then again I'm pretty satisfied with the more streamlined approach to Fighters from the core book.