What are the Win10 system versions of Microsoft?

The Win10 system technology preview has been released for nearly a month. Microsoft is currently compiling the new Windows 10 version every day, but only a few versions are pushed to the public, which is exactly two of the hundreds of builds. The following small series for everyone to introduce.
The question then comes, how does Microsoft decide which versions will be available to public users for download.

Microsoft's Win10 development department has more than one. Different departments are responsible for different issues. For example, the fbl_release department is responsible for testing and pushing the version to the public, while the main department that develops Win10 functions is the Winmain department.
Winmain developed a new version one by one that will be sent to fbl_release for testing, but even if fbl_release continues to get new versions, it does not mean that users will get upgrades in the first time.
fbl_release has a set of body push schemes, called Flight Level, which contains two levels, high level and low level. The low level build version will be pushed to the public for testing. The other high level will also provide users with downloads, but not for mass users, but for Microsoft partners.
fbl_release gets a new version every day, but most versions of the Flight Level are HIGH, and very few Build versions with the label LOW will be available for download to the general public.
Therefore, the problem is coming again. In the "Computer Settings", in the Preview Build tab of the Update and Recovery page, the two options Fast and Slow for getting the new version speed are going on. Very simple, the Build version of these two options is the Low level version in the Flight level, set to Fast will get the Low level version push as fast as possible, and set to Slow will also get the same version, but the acquisition speed will be Slower.