This option is only offered, if the wrapped VST plugin actually can save (and load) bank files.
This usually is the case, if it reports 'No of Progs' to be more than 1, often 128 - but might also be 64, 32, 100 or what not.
When a virtual bank gets saved, it should have that number of presets in it.
If the virtual bank to be saved has more presets, then the presets at the end of the virtual bank will not be saved.
If the virtual bank to be saved has less presets, then the last preset of the virtual bank will be used to fill up the remaining program slots.
For this case, it is recommended to add a preset like 'Init' or 'Default' or something like that to the end of a virtual bank.
Therefore - if you intend to save a virtual bank - it might be good to be aware about how many programs the wrapped VST plugin will use for saving of banks.
This can be looked up in 'Menu'->'Details to "[name of wrapped VST plugin]"...' under 'No of Progs'.
If you use context menu entry 'Bank (virtual)' -> 'Add to' in the 'Presets Pane', it is ensured, that a preset will not be added to the virtual bank
in case it is already there. This way, it can be avoided that virtual banks contain duplicates.
But with drag'n'drop of selected presets from the 'Presets Pane' to a virtual bank in the banks section of the 'Filter Pane', no check for duplicates will be done
and thus with this method it is easily possible to add duplicate entries to a virtual bank.
This behavior is intentional, because sometimes one may want to be able to have duplicates.
E.g. a 'pseudo' preset named like '---------------' could be useful more than once to create sections within a virtual bank.
If there are duplicates in a virtual bank, then - when it comes to removing of presets from it - there is another thing to be considered:
If exaclty one virtual bank is selected in the banks section of the 'Filter Pane' and no other filter is selected/active there,
then removing of one or more selected presets will exactly remove those presets from their positions in the virtual bank.
But if any other filter is selected/active (e.g. tag 'Bass' from the 'Category' tag group) and now a displayed preset should get removed from the virtual bank,
then - if there would be duplicates - all presets with this name will be removed from the virtual bank.