This document serves two purposes. The first is to document the installed directory structure and the format and purpose of each file. The second purpose is to document how APT views the Debian archive and deals with multiple package files.
The var directory structure is as follows:
/var/state/apt/
lists/
partial/
xstatus
userstatus
cdroms.list
/var/cache/apt/
pkgcache.bin
srcpkgcache.bin
archives/
partial/
/etc/apt/
sources.list
apt.conf
/usr/lib/apt/
methods/
cdrom
ftp
http
file
gzip
copy
As is specified in the FHS 2.0 /var/state/apt is used for application data that is not expected to be user modified. /var/cache/apt is used for regeneratable data and is where the package cache and downloaded .debs go.