From https://blogs.technet.microsoft.com/windowsitpro/2017/07/27/waas-simplified-and-aligned/:
“As we announced back in April, Microsoft is aligning our servicing models with twice-per-year feature update releases targeting March and September, and 18-month servicing timelines for each release. While the first fully-aligned release will occur later this year with the Windows 10 Fall Creators Update release and a corresponding Office 365 ProPlus release, we got a head start with the Windows 10 1703 release (a.k.a. Creators Update): It marks the first of our semi-annual releases, each of which will be serviced for 18 months.
As part of the alignment with Windows 10 and Office 365 ProPlus, we are also adopting common terminology to make it as easy as possible to understand the servicing process. The two most important terms to understand:”
Also see: Microsoft rationalizes and rebrands Windows 10, Office updates again.