This window allows you to change the coordinate or data units of the current spectrum to a limited number of preset values. You can only change units when the current spectrum already has a correctly set coordinate system or data units.
If you need to set the coordinates or data units you must use the two toolboxes available in the main window. These also offer considerably more flexibility, particularly when setting spectral coordinates.
If you do not want to change the units of an axis, just leave the value set at "Unknown".
When the plot is displaying more than one spectrum, only the current one is actually modified. If you want the other spectra to be drawn as if in the same coordinates or data units, then you must select the menu item Options->Match coordinates and/or fluxes, but this will only work when all the displayed spectra have proper spectral coordinates and data units defined.
The SideBand: control is for switching between sidebands (upper and lower) when display spectra that are derived from a dual sideband instrument. This is not available when your spectra do not have this feature. When displaying a dual sideband spectrum the inactive sideband coordinates will be displayed along the unused X axis (usually the top).