[Dxbase] HF Radio QSY Question

[Ed Parish K1EP]

When you bring up the Tools->Set HF Frequency (ALT-Q) window, a screen appears with all of your saved frequencies.   The columns are: Comment, VFOa, VFOb, Mode, and Spare.  The Spare column has a check box.  I have searched the Help files and can't find out what Spare does.  I can set the check box or not set it, but what does it do???  

(I won't ask the question, why is the function labelled Set HF Frequency, when I can set a VHF frequency on my 6M [or 2M or 70cm] equipped radio...)