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A general description is here.
‘R CMD check’ now gives a warning rather than a note if it finds inefficiently compressed datasets. With ‘bzip2’ and ‘xz’ compression having been available since R 2.10.0, it only exceptionally makes sense to not use them.
The environment variable ‘_R_CHECK_COMPACT_DATA2_’ is no longer consulted: the check is always done if ‘_R_CHECK_COMPACT_DATA_’ has a true value (its default).