Microsoft is changing the rings and the development model

OR Microsoft changed the format of the names he 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 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.

The Insiders on the channel Dev will download early builds in the development cycle that will contain the latest code under development by our engineers. These versions will not correspond to a specific version of Windows 10.
The channel Beta will be ideal for those who want to try the new version. Insiders and IT professionals on the channel Beta will be able to control their upcoming capabilities Windows 10, while at the same time they will receive 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 channel Preview release will have access to the upcoming version of Windows 10 before being released to the world, with all the advanced quality updates and some key features. These versions are supported by Microsoft.

OR Microsoft also mentioned that they may use new channels for more Insiders experiences.

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

If you are currently using an Insider preview ring, the 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.

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

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