files necessary to obtain/repair a system ("root").
binaries for single user mode, for all users.
files necessary to boot a system.
system-wide configuration files.
users' home dirs (not "your home directory"!).
libraries for binaries.
other mounted filesystems.
temporarily mounted media.
additionally installed software.
process and kernel stati.
home directory for user root.
system binaries (for root mainly).
files provided by services running on the host.
secondary hierarchy for user data.
tertiary hierarchy for user data.