When Microsoft opened up Xbox Insider Program to everyone, it created four rings: Alpha, Beta, Ring 3, and Ring 4, the former three of which require some effort to get into, which will earn you an invitation. Up until now, only the Alpha and Beta rings have received preview builds of the Creators Update, despite the company populating Preview Ring 3 well over a month ago.
That all changes today, as 'a subset' of Ring 3 Insiders are getting build 15055.1000 tonight, which was offered to the Alpha ring four days ago. Microsoft says that the remaining users in the ring will receive the build in the coming weeks.
In other words, if you thought that four separate rings were complicated, it just got even more so. Let's break this down a bit; some Preview Ring 3 Insiders got it tonight, and some didn't. The rest will get it over the coming weeks, yet the Creators Update is going to roll out in April. After all, the version number is 1703, indicating a March RTM.
Judging by the pluralization of 'weeks', we can assume that this rollout will be completed by no earlier than March 29. Presumably, at some point it will also graduate to Preview Ring 4, which is similar to the Release Preview ring of the Windows Insider Program.
Also, to be clear, Microsoft did say that the Creators Update will come to PCs first and to other platforms later. Other platforms presumably includes the Xbox One; however, the firm said the same thing about the Anniversary Update, despite rolling it out to the console days before PCs.
If you're on Alpha or Beta and you wish to receive builds less frequently, you can downgrade your console to Preview Ring 3 through the Insider Hub. All three rings are now on the same build. If you're on Preview Ring 4, you can work your way up by completing Quests and Surveys in the Feedback Hub.