Approximately 5 years ago Microsoft started rolling out test builds to the Windows Insiders in the Fast Ring, and those test builds were then rolled out to Slow Ring and Release Preview Ring. Over the years, Microsoft adjusted how they have released the Windows 10 Preview builds by using the ring model.
Now they are transitioning and converting the current Windows Insiders ring model, based on the frequency of builds, to a new channel model. In addition to this change, Insiders will also find similar offering names and expectations across Windows and Office programs and soon Microsoft Edge and Microsoft Teams.