To help you visualise where we are in terms of the window for these monthly OS updates, here's a helpful (revised) chart:

Fall Creators Update ('rs3', i.e. 'Redstone 3', at least in name) is the 'production' branch of Windows 10 Mobile, and 'Patch Tuesday' fix round-ups for it continue, with the official list for 15254.401, the '2018-05' update', mainly composed of minor fixes and security patches, at least in terms of mobile relevance.

From the official release notes:

• Security updates to Microsoft Scripting Engine, Microsoft Edge, Microsoft Windows Search component, Windows Kernel, Device Guard, Windows storage and file systems, and the Common Log File System driver.
• Updates for Microsoft Edge to respect the video preload flag in certain scenarios.
• 'Quality Improvements'. 

As usual and as per the graphic above (the rectangles in a shade of red), at least one other Windows 10 Mobile branch is getting an update too, so all those older phones (Lumia 930, 830, 735, 640, etc.) finishing life on the Creators Update (if the user was smart enough to use our (ahem) hack!) aren't losing out on the core security fixes, at least.

As usual, in today's vulnerability-stricken, malware-strewn and fraudulent times, we should never disregard security fixes (e.g. to the Edge browser) though, so update today in all affected mobiles.

In each case you get the core security and component updates, though obviously without any subtleties in functionality introduced in later branches. As ever, with all phones, head into Settings/Update & Security/Phone update in order to get the update immediately.