This blog is updated daily.
A general description is here.
There are two new environment variables which control the defaults for command-line options.
If ‘R_WIN_INTERNET2’ is set to a non-empty value, it is as if ‘--internet2’ was used.
If ‘R_MAX_MEM_SIZE’ is set, it gives the default memory limit if ‘--max-mem-size’ is not specified: invalid values being ignored.