Maptitude workspaces typically contain many different layers, all of which are listed in the Display Manager. The software needs a way to determine which layer to act upon when using a tool, so Maptitude has a “Working Layer” which specifies that chosen layer as the one for Maptitude to act upon.
The working layer is the layer in Bold in the display manager, as well as the listed layer in the upper-left pulldown. You can change the working layer by right-clicking your desired layer in the display manager and choosing “Make Working Layer”, or by choosing that layer from the pulldown.
Note: The highlighted layer does not mean anything.
For example, if you wanted to color all of the US states by population, you would make the State layer your working layer and go to Map>Thematic Mapping>Color and select population as the field. This way, Maptitude knows that you want to color the State layer and not some other layer.