What is the difference between the ring, brush and combined versions?

They perform three related, but distinct, jobs.

The ring version organises cables travelling across the front of the rack. The brush version provides a controlled passage between the front and rear.

The combined version does both, placing rings directly in front of the brush openings so a cable can emerge and immediately join the horizontal route.

There is no hierarchy in which the combined version is automatically "better"; it simply avoids using two separate pieces of hardware when you genuinely need both functions.