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A general description is here.
‘tools::Rdiff(useDiff = TRUE)’ checks for the presence of an external ‘diff’ command and switches to ‘useDiff = FALSE’ if none is found.
This allows ‘R CMD Rdiff’ to always work.
‘R CMD check’ can optionally (but included in ‘--as-cran’) validate the HTML produced from the packages ‘.Rd’ files. See <URL: https://blog.r-project.org/2022/04/08/enhancements-to-html-documentation/>: this needs a fairly recent version of HTML Tidy to be available.