Difference between UI and Visual Design/Style Guide?

From looking at some of the style guide for web applications that are available, there is a distinction made between the visual (icons, colours, logos, etc) and UI (look and feel of user interface elements) style guides.

There are some overlaps between the two since visual elements also form part of the UI, but in terms of the way content (e.g. labels, messages, fonts, etc) is presented on the UI, are there gaps that are not adequately addressed in the visual guide that needs to go into the UI style guide, or is a hybrid combination of the two best for web applications (not websites)?

Is this still a relevant comparison since these days organisations are tending to move towards design languages or system that combine the two more closely than previously before?