Un-masked Copper Boundary across RF section
You meant Via Stitching.

In RF application, you have a lot of current flows happening that behave quite differently than in non-RF application, and can take some really weird ways.

This is to reduce this effect.

The via stitching is there to reduce high frequency current flow going though the internal layers, don't forget a PCB is a dielectric, but still conduct somewhat electricity, or charges, especially in high frequency application.

The traces are uncovered for the same reason, avoiding having current flow flowing above the solder mask and messing with the rest of the circuit.

Probably on top of these ring, a metallic housing might be soldered, acting as a Faraday cage and avoiding having EMI being emited by the circuit. 

Like this:
 



This doesn't apply to your question, but it is an interesting addition:

These types of uncovered rings are also sometimes used in acquisition when the signal carries nA level currents. 

Called Guard trace, they usually feed a follower op-amp, and the ring is placed around the signal trace and connected to the output of the op-amp, bringing the ring to the same voltage as the trace and thus avoiding leakage current. The cutout are for the same reason