From the Acad Help file.
Hope this helps..A drawing can use either named or color-dependent plot styles, but not both.
CONVERTPSTYLES converts a currently open drawing from color-dependent plot styles to named plot styles, or from named plot styles to color-dependent plot styles, depending on which plot style method the drawing is currently using.
For example, a drawing using color-dependent plot styles assigns plot properties to objects and layers by color. In other words, all objects with the same color have the same plot properties. CONVERTPSTYLES converts the drawing to use named plot styles that can be applied to objects or layers independent of color assignment. In other words, all objects with the same color can have different plot properties.
When you convert a drawing, CONVERTPSTYLES sets thePSTYLEMODE system variable to the appropriate setting (1 = named, 0 = color-dependent).