Required content settings can be configured to do more complicated
things than just setting the required number of [[copies]] of your data.
For example, you could require that data be archived in at least two
archival repositories, and also require that one copy be stored offsite.

The format of required content expressions is the same as
[[preferred_content]] expressions.

Required content settings can be edited using `git annex vicfg`
or set using `git annex required`.
Each repository can have its own settings, and other repositories will
try to honor those settings when interacting with it.

While [[preferred_content]] expresses a preference, it can be overridden
by simply using `git annex drop`. On the other hand, required content
settings are enforced; `git annex drop` will refuse to drop a file if
doing so would violate its required content settings.
(Although even this can be overridden using `--force`).

Also, `git-annex fsck` will warn about required contents that are not
present.

