Microsoft this week released the second upgrade for Office 365 commercial subscribers on the slow train, and warned those still running the original Office 2016 applications that they have four more months before they will be required to update.

Alongside a large number of Windows security updates issued Tuesday, Microsoft also released build 1602 of the Office apps to corporate Office 365 subscribers who hew to the "Deferred Channel" track.

Deferred Channel is the slower of the two main release tracks Microsoft established for Office 365. (Until February, it was called "Current Branch for Business" to match the name of a slow release track for Windows 10.) Unlike the faster "Current Channel" (CC), which boasts monthly updates to the Office 2016 applications -- Word, Outlook, Excel and the like -- Deferred Channel (DC) only provides updates every four months.

Consumers who subscribe to Office 365 Personal or Office 365 Home are on the CC track; they don't get a choice. IT administrators managing commercial Office 365 plans, such as Business Premium, E3 and E5, may select either CC or DC for some or all users.

The slower pace of DC is designed to give organizations time to test the updates before deployment, and essentially sets those on the CC track as testers for DC customers. Microsoft follows the same practice with Windows 10, using consumers as a large test group to identify, and fix, flaws or compatibility issues, before the operating system's major upgrades reach the more important corporate customers.

So far, Microsoft has issued two DC updates: One in February 2016, the second this month. The two were labeled 1509 and 1602, respectively. Build 1509 was the original release of the Office 2016 client applications, which, as its nomenclature indicated, was the original launch code shipped in September 2015 to consumers and companies on the Current Channel track.

Office 365's applications are on an every-four-months schedule for DC, with updates slated to release in February, June and October of each year. The cadence is important for customers to remember because Microsoft has set rules for Office 365 subscribers who adopt DC. They may skip an update, but no more than one, sticking with an individual Office 2016 feature set for no longer than eight months.

For example, those running 1509 -- again, the original versions of the Office 365 apps -- since February must apply 1602 or later by October. If they do not, the apps will no longer receive security patches and other fixes.

"The existing Office 365 ProPlus version (1509) within the Deferred Channel will continue to be serviced for an additional four months," Amesh Mansukhani, a senior program manager on the Office engineering team, said in a post to a company blog Tuesday.

In October, the DC will receive the 1605 update, the same one issued to CC customers this week.

The rolling release tempo for Office 365's Deferred Channel allows corporate customers to pass on one update. In this opening scenario, where DC users have been on 1509 since February, they will be allowed to skip 1602, but not both 1602 and 1605. In October, they may upgrade to either 1602 or 1605. The former falls off the support list in February 2017, while the latter will be "serviced" -- to use Mansukhani's term -- until June 2017.

Yes, it can be confusing.

This story, "Microsoft starts clock ticking on Office 2016's first upgrade" was originally published by Computerworld .