Phoebe,
Classes aren’t loaded unless you specially ask for them.
EE allows you to load a lot of commonly used ones like this:
global $DB, $SESS;
Otherwise you will want to call the class yourself within the function that needs it. EE does provide some ways to make this more reliable.
PATH_MOD will give you the full path to the modules directory for instance. So your extension will be able to find the correct full path to the module easily.
An example of calling a class from the Plugin directory can be seen in this thread. Doing it from a module would work very similarly except you would use PATH_MOD and a module name instead of PATH_PI and a plugin name.
Jamie
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