An index map is a tiny quadratic overview map showing a red rectangle of the plot area in a regional setting with coastlines and lines of quadrants or something else. Available on scaled plots only.
The Index Panel with help panel
Read the in-panel documentation, and click the Example button to understand the usage of the Index Map. Try to set up the sidelengths etc. correctly, then press the Setup-view button to get a preview of the index map in Geocap's grpahical window.