Variable: org-highest-priority
org-highest-priority is an alias and customizable for
org-priority-highest, defined in org.el.gz.
Value
65
Documentation
The highest priority of TODO items.
A character like ?A, ?B, etc., or a numeric value like 1, 2, etc.
The default is the character ?A, which is 65 as a numeric value.
If you set org-priority-highest to a numeric value inferior to
65, Org assumes you want to use digits for the priority cookie.
If you set it to >=65, Org assumes you want to use alphabetical
characters.
In both cases, the value of org-priority-highest must be
smaller than org-priority-lowest: for example, if "A" is the
highest priority, it is smaller than the lowest "C" priority:
65 < 67.