Microsoft has announced some changes to how the Windows Insider Programme operate with the focus on testing more experimental technologies with Preview users.
Microsoft will use the Dev channel to incubate new ideas, with the intention that the new features show up in future Windows releases, but also the real possibility that they will never ship.
Due to this, Microsoft will be doing more A/B testing, and there may be variations on features which are never documented due to this.
This means the Dev and Beta Channels are better seen as parallel active development branches but previewing different things.
The Windows Insider Dev channnel
The Dev Channel will be for long lead work from Microsoft’s engineers with features and experiences that may never get released as they try out different concepts and get feedback. Builds released to the Dev Channel should not be seen as matched to any specific release of Windows and the features included may change over time, be removed, or be replaced in Insider builds or may never be released beyond Windows Insiders to general customers.
The Windows Insider Beta channnel
The Beta Channel will be for previewing experiences that are closer to what Microsoft will ship to general customers. Because the Dev and Beta Channels represent parallel development paths from their engineers, there may be cases where features and experiences show up in the Beta Channel first.
Microsoft will deliver updates to features and experiences in builds from the Dev and Beta Channels by releasing Feature, Web, and Online Service Experience Packs on top of these builds too.
The Windows Insider Release Preview channnel
The Release Preview Channel will deliver features coming to general customers soon.
New Windows Insider Programme icon
Microsoft also announced a new Insider Programme icon symbolizing the three channels.

Insiders in the Dev Channel will soon see this icon showing up under Settings > Windows Update > Windows Insider Program. The icon will also be showing up in emails and other areas.
Switching from the Dev to Beta channel
Microsoft says they will soon offer the opportunity for Insiders to switch from the Dev to Beta channel if they prefer, with the details coming in the next few weeks. This is not usually possible without re-installing.