Microsoftchristophergg- There is only one Semi-Annual Channel version, which is released each 6-months. This one is this April 2018 Feature Update or 1803, and is the fifth of the Feature Updates.
SAC (targeted) is how we say the customer should deploy the update to their validation devices. They will have several rings typically. The first is the preview rings (for devices enrolled in the Windows Insider Program) and this gives 6-months or so to prepare (test and validate) for the SAC release 1803. Then the next ring is to validate the features work as planned, and there are no issues in production. Then once this has been validated they deploy to the next ring (wider ring, production ring, etc) and they may have several rings if they want to scale across large companies, or different business units etc. Hope that helps.
https://blogs.technet.microsoft.com/windowsitpro/2017/07/27/waas-simplified-and-aligned/
