Why can you check plot and not check save in ADEXL? Shouldn't it be automatically saved when you plot? If this is the case, the save checkbox should be automatically checked and/or disabled. Or have I misunderstood the meaning of plot vs save?
If the output is an expression, then the expression results do not need saving - they are computed from other saved signals. So the only choice is whether something is plotted or not.
If the output is a signal, then it can be saved (by the simulator) but not plotted - for example, you might want to plot it later, or use the value in an expression - which you then want to plot.
Put another way, the save checkbox controls whether the simulator saves that thing or not, and the simulator is not saving expressions, only signals.
If it's an expression, the reason for being able to plot but not save is as I said in the beginning - there's nothing to be saved because the expression is handled by the ADE XL post-processing and saving it is meaningless.
If it's a signal, you could plot it without saving if save all is enabled and covers the signal that you were going to plot (but you'd still have the choice of saving or not - but it may make no difference because it may have saved the signal anyway).