Microsoft began testing out Windows 10 20H1 back in February, offering it to those in the Skip Ahead subset of the Fast ring. For the first time, the firm was testing a version of Windows that was a full year out, with an entire update in-between. Months later, the Slow ring began testing out Windows 10 19H2.
But 19H2 is ready to ship as the Windows 10 November 2019 Update. That means that it's time to promote 20H1 to the Slow ring, and that's exactly what Microsoft is doing today. If you're on the Slow ring, you can check for updates now and find Windows 10 Insider Preview build 19013.
Build 19013 was first released to the Fast ring two weeks ago, and it was actually serviced later in the week. The one being offered to the Slow ring is indeed a serviced build as well, only fixing some bugs that put it above the higher bar set by the Slow ring. Of course, being that this is the first 20H1 build in the ring, you'll also get all of the other new features that the Fast ring has been enjoying.
Windows 10 20H1 is pretty close to being feature-complete, if it's not already. The final build might be ready for Insider testing as early as December, followed by a public release a few months after that. In the meantime though, you can test it out in the Slow ring.