file_usage_add: custom type for my module
Yes, type just needs to be something unique within your module's types of file usage. The value of the string itself is arbitrary.

So if you have a module which stores pictures of cats and dogs, and you want to address those two different types of usage separately, you might use (contrived examples obviously):

$cat = MYMODULE_load_cat('whiskers');
file_usage_add($file, 'MYMODULE', 'cat_pictures', $cat-&gt;id);

$dog = MYMODULE_load_dog('fido');
file_usage_add($file, 'MYMODULE', 'dog_pictures', $dog-&gt;id);


But the type for those could just as easily be cat_foo and dog_foo, all that matters is that you (the module developer) know what they are, so they can be used appropriately with file_usage_delete().