From the manufacturer

MSX adds visible light details to thermal images

FLIR MSX (Multi-Spectral Dynamic Imaging) adds visible light details to thermal images in real time for greater clarity, embedding edge and outline detail onto thermal readings. Unlike image fusing (merging of a visible light and thermal image), MSX does not dilute the thermal image or decrease thermal transparency.

Thermal imagers with MSX capabilities actually have two cameras—a thermal imager and a visible light camera (i.e. a typical digital camera). Every time you take a picture, both a thermal image and a visible image are captured simultaneously. Built-in software extracts key visible image elements—outlines, words, numbers, and other high-contrast edge details–and adds them to the thermal image. This helps give the image definition and makes it easier to tell what you’re looking at.