I was wondering how many states a checkbox has.

Based on https://orgmode.org/manual/Checkboxes.html I conclude that there are 3 states:

1. [   ] - not done

2. [ - ] - an intermediate state

3. [X] - done

So I think all 3 states should be available to choose from in the function 'org-columns-next-allowed-value'' 

for functions that use checkboxes. 

In code this is a line https://git.savannah.gnu.org/cgit/emacs/org-mode.git/tree/lisp/org-colview.el#n754

So I would change it from this:

-      (`(,_ ,_ ,_ ,(or "X" "X/" "X%") ,_) '("[ ]" "[X]")))

to that:

+     (`(,_ ,_ ,_ ,(or "X" "X/" "X%") ,_) '("[ ]" "[-]" "[X]")))

What do you think? 

Regards,

Sławomir Grochowski