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A general description is here.
There is a combined installer (called ‘R-<version>-win.exe’). When used under 32-bit Windows this works as before and installs 32-bit R. When used under 64-bit Windows there are options to install either or both of 32- and 64-bit R: the default is to install both.
The default package type is ‘"win.binary"’ on both 32- and 64-bit builds: a single repository contains binary packages for both architectures. This is in place for CRAN, CRANextras and BioC: ‘type = "win64.binary"’ can still be used for any repositories which follow the R 2.11.x convention. With the default filters (see ‘?available.packages’) packages will only be offered if they are available for the current architecture.
The default personal library, e.g. ‘~/R/win-library/2.12’ is now the same for both 32- and 64-bit R.