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A general description is here.
‘summary()’ behaves slightly differently (or more precisely its ‘print()’ method does). For numeric inputs, the number of NAs is printed as an integer and not a real. For dates and datetimes, the number of NAs is included in the printed output (the latter being the wish of PR#14720).
The ‘"data.frame"’ method is more consistent with the default method: in particular it now applies ‘zapsmall()’ to numeric/complex summaries.