Microsoft is changing the rings and the development model

Microsoft has changed the format of the names it uses in Insider Rings in Windows. The new names reflect the quality of the builds instead of the traffic frequency.

According to the official announcement

We are changing and transforming the current model of Rings, which was based on the frequency of releases, into a new model with channels that will be based on the quality of the releases and will better support the parallel coding efforts. In addition to this change, Insiders will find similar names in Windows and Office development programs and soon in Microsoft Edge and Microsoft Teams. Our goal is not only to make it clearer for existing Insiders to choose the experience that suits them, but also for new Insiders to choose the right channel when participating.

So later this month, the Fast ring will become the Dev Channel, the Slow ring will become the Beta Channel, and the Release Preview ring will become the Release Preview Channel. The new name scheme is reminiscent of the format used by Microsoft to develop the Edge browser.

Insiders on the Dev channel will download early builds in the development cycle that will contain the latest code under development by our engineers. These versions will not be assigned to a specific version of Windows 10.
The Beta channel will be ideal for those who want to try the new version. Insiders and IT professionals in the Beta channel will be able to check the upcoming features of Windows 10, while receiving relatively reliable updates that will be validated by Microsoft. These releases will be linked to a specific upcoming release.
Insiders and IT professionals on the Release Preview channel will have access to the upcoming version of Windows 10 before it is released to the world, with all the advanced quality updates and some key features. These versions are supported by Microsoft.

Microsoft also said it may use new channels for more Insiders experiences.

Microsoft will use the same names for Office Insiders. For Office versions, there will be Beta and Release Preview channels, but without the Dev channel.

If you are currently using an Insider preview ring, Microsoft will automatically change your package based on your current ring to the new channel that corresponds best. You will not need to change anything.

• Fast ring goes to Dev Channel
• the Slow ring goes to Beta Channel and
• the Release Preview ring goes to the Release Preview Channel

Insiders can always change these settings from Settings - Update & Security - Windows Insider.