First things first: Customers who use Windows Update and Automatic Update are not impacted by any of these changes. The changes Microsoft is making are for those who get updates from Windows Server Update Services (WSUS) or other business update-management tools.
Since October, Microsoft has been making available a Monthly Rollup and a Security Only update available to IT pros on Patch Tuesday each month, and a Preview Rollup on the following Tuesday.
As of December 2016, Microsoft stopped making available Security Only updates for PCs where Monthly Rollups from the same month or later month were already installed. Microsoft went back and revised its Security Only updates for October and November 2016 to comply with this new model.
That's one change. Microsoft also is changing how Internet Explorer 10 and 11 are patched and updated.
Before October 2016, Microsoft made updates for IE10 for Windows Server 2012 and IE 11 for Windows 7 Service Pack 1, Windows 8.1, Windows Server 2008 R2 and Windows Sever 2012 R2 available as separate monthly updates. Then, from October 2016 to January 2017, Microsoft made any IE fixes for the given month part of the Security Only Update. Because some business users wanted smaller package sizes for their patches, they requested choices about how to get their IE fixes.
In response, starting with the February 2017 patching schedule, the Security Only update won't include updates for IE, and IE fixes and updates will be available as a separate update. The Monthly Rollup will continue to include updates for IE as a single update, however, so those relying on these monthly rollups don't need to install IE updates separately.
There's a handy-dandy Microsoft chart, embedded above in my post (and also viewable here), that may make these tweaks a bit clearer.