There are 4 distinct alignment override modifiers:
:alT :alB :alL :alR
which correspond to setting the flow of dates alignment to top, bottom, left and right, respectively.
Adding one of these modifiers is the equivalent of setting one of the following flow of dates minicalendar options for the minicalendar to which this modifier belongs.
If you use an alignment override setting that is impossible (such as bottom aligning a vertical minicalendar), then a warning message may not be issued during diary generation, and it is up to you to identify and correct the issue.