Those are slide files - pictures of dwgs. Draw a circle inside a rectangle and then type Mslide. You will then get a picture of the rectangle with circle. Redraw closes a slide. Look up in Help.
In old versions that used the menus (pre ribbon days) the sld files were used to display images of parts, blocks, line tyes, hatch types and etc. You could also use them in the slide player to create an animation of a drawing. I used to make a group of them years ago to use as a screensaver in the dos days.