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A general description is here.
The LaTeX style for producing the PDF manuals, ‘Rd.sty’, now loads the standard ‘amsmath’, ‘amsfonts’ and ‘amssymb’ packages for greater coverage of math commands in the Rd \eqn and \deqn macros. The \mathscr LaTeX command is also provided (via the ‘mathrsfs’ package, if available, or the ‘amsfonts’ bundle otherwise), fulfilling the wish of PR#18398.