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.
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.
Topic 178260, last updated on 23-Jun-2020