Modules can define other modules as dependencies. You should do this
whenever you require resources (like database tables) that belong
to module A in module B, if you want to use automatic css/js
inclusion (see javascript_css doc), or if you just feel like your
module should not be accessible if another module is not present.

"baseconfig_partitions_cdn" has "baseconfig" as a dependency; it
does not directly access any tables of the module "baseconfig", but
since it depends on the getconfig-API of the baseconfig module, one
might argue that having the partition setup feature is of limited
use if the clients can't fetch it.

"statistics" module has "js_chart" as a dependency, since it
makes use of the automatic clientscript.js inclusion. The statistics
module uses the chart javascript library to draw, well, charts.


You can write modules that can optionally interact or take advantage
of other modules if they are present. The statistics module can
make use of the locations module to show which location a client
belongs to, but it would still work without the locations module
being present. You can check for the availability of a module
by calling Module::isAvailable('<module_name>')
If the module is available, the call will return true, and
the class autoloader for the requested module is also installed,
so you can use any classes from ./modules/<module_name>/inc/*.inc.php
Again, see how the statistics module calls Location::getFromIp()
and other functions after calling Module::isAvailable().
If you'd try to access Location::* without doing so, you'd get
a class not found error.