Its successor will be built in March 2018, hence named 1803. They get two major servicing updates each year, with each version named with a two-digit year, two-digit month the current version is 1709 because it was built in September 2017.
Mainstream Windows, Office, and Windows Server users are on the Semi-Annual Channel (SAC). After a period of refining the details, Microsoft settled on the current scheme. With Windows 10 and 'Windows-as-a-service,' that policy got all shaken up. That was followed by five more years of extended support, during which time it received security updates only. Each release had five years of mainstream support, during which it received security updates, feature improvements, and stability fixes. In the olden days, Microsoft's support policy for Windows and Office was simple.
