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A general description is here.
R now supports custom binary types which are of the form ‘"<system>.binary.<build>"’ where ‘<system>’ is lower-case name (letters only) of the system and ‘<build>’ is the name of the build (alphanumeric and dashes). The corresponding paths returned by ‘contrib.url’ (which govern repository layout) with the above ‘type=’ have the form ‘bin/<system>/<build>/contrib/<x.y>’ where ‘<x.y>’ is the R version without patch level. This is a generalisation of the binary types previously supported on macOS. In additon, binary packages can have the extensions ‘.tar.bz2’, ‘.tar.xz’, ‘.tar.zst’ and ‘.tar.zstd’ with the corresponding compression support.
The value of ‘.Platform$pkgType’ which defines the native binary type for the current build of R can be overridden with ‘R_PLATFORM_PKGTYPE’ environment variable (previously this was only set by the macOS builds of R). This should be typically only done by the author of a binary R distribution to define the type of binaries supported by that build of R.
When installing binary packages the time stamp in the ‘"Built"’ field is considered even if the package version is identical to allow binary package repositories to update binaries without the need to increase the package version.