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A general description is here.
‘R CMD check’ now tries re-building all vignettes rather than stopping at the first error. Whilst doing so it adds ‘bookmarks’ to the log. Optionally (see ‘R Internals’) it can re-build each vignette in a separate process.
It now checks for duplicated vignette titles (also known as ‘index entries’): they are used as hyperlinks on CRAN package pages and so do need to be unique).