Descriptions can currently only be accessed directly via the Entity data structure. This means they are tracked using the X aspect for now; we may in the future track which part of the Entity data structure is actually used, compare T151717. When we do that, we should track description usage separately.
Similarly, when formatting a statement for the #statements parser function, the formatter may access the description in addition to label (L) and target page (T). That access to the description should be tracked separately, not as X, O, or D, to avoid redundant purging when descriptions are updated.