The Photoshop Preference Scale UI does nothing on windows 10 Creator's Edition and CC 2018.1 19.1 IMO the preference should simply be removed. However, that may serve some function on other version of Windows Adobe noted nothing about the other version of windows in their 19.1 announcement only Windows 10 Creator's edition. Set your windows displays scaling for each of the displays you want to scale. Photoshop CC 2018.1 19.1 will scale its UI element on those Displays set to some 25% increment over 100% up to 400%.
Here is what I read
Windows High-Density Monitor Support
With this release, Photoshop on Windows 10 Creator’s Edition now offers a full range of choices for UI scale factors from 100% through 400%, in 25% increments. This means that the Photoshop user interface will look crisp, beautiful, and the right size no matter the density of your monitor. Photoshop will now automatically adjust itself based on your Windows settings, making it simple to set up.