One of the key data structures using by cran2deb is commonly called 'pkg'.
It is constructed in R/getrpkg.R by prepare_pkg. prepare_pkg obtains
an R package and converts the source package into something suitable for use
with Debian.

If a particular upstream version has already been used to create a Debian
package, then the source tarball of that upstream version is expected to be
available locally, and is used for building. In this case no conversion is
performed, so the archive does not change. In future it may be desirable to
obtain the source tarball from some central archive but this is not done at the
moment.

download_pkg in R/getrpkg.R obtains the tarball (archive) of the R package, either
from the previous Debian build, or from the R archive. The field pkg$need_repack
indicates if the obtained archive must be repacked into a form acceptable
as a Debian source archive. This repacking, if necessary, is performed by
repack_pkg in R/getrpkg.R


Most of the creation of pkg is done by R/getrpkg.R. However some more build
specific operations (such as determining the new build version pkg$debversion)
are performed by R/debianpkg.R.