i would like to be able to have the choice to select only one 'category' or all the 'categories'.

i do so:
if user nothing select in list that he selected all-
...
if isnull(me![LIST_NAME]) then
'all
else
'something
end if
...

or you may use special function to fill list with first value-all. see help.
glad to answer

can you please expound on all that you have coded.