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 defining spectral coordinates.
If you do not want to change the units of an axis, just leave the value set at Not set or unrecognised.
When you see the value Not set or unrecognised, that means that the current spectrum has no properly defined coordinates or data units for that axis (as appropriate), or, that the settings do not match one of the pre-defined set that you can switch to. Note that data units which match one of the preset values dimensionally will be displayed in the preset format, so may not match values shown elsewhere, say in the plot labels.
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 make sure that the menu item Options->Match coordinates and/or fluxes is selected, 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). You can also use this control to display coordinates offset from the Local Oscillator value.
The Origin: control allows you to display offset coordinates. In this case you can select to have no offset (None), use the existing offset, if defined (Default, usually the same as None), or display coordinates offset from the rest frequency. Note for velocities this last option isn't useful as they are already displayed offset from this value. If you want to set another value as the origin then you need to use the advanced toolbox available in the main window.
The Standard of rest: control allows you to switch between the various supported rest frames. If the current spectrum doesn't support these the value will be set to Not set or unrecognised.
The Rest Frequency: field displays the current rest frequency in GHz. This can be changed in two ways, either by selecting a position along the plot X axis, which you start by pressing the Pick pos button and then clicking on the plot, or by selecting a line identifier, which you do by pressing the Pick ID button and clicking on the plot in the vicinity of the desired label. When picking a line identifier the rest frequency is set to that of the line, not of the position that it currently occupies on the plot (these will differ if the spectrum has a source velocity).
SPLAT-VO -- A VO-enabled Spectral Analysis Tool