What is the deal with config collections? When manipulating configuration yaml files in config/sync, I note that if you put files in subdirectories, and run drush config:import, the files in those subdirectories are denoted as belonging to a 'collection'. (Top-level files have no collection listed)

This seems like it might be a useful organisational unit. However, it seems like configurations that are part of a collection are not acted on.

For example, if I put a node.type.test.yml in config/sync, and run drush config:import then a corresponding node type is created as expected. However, if instead I put the file in config/sync/test and run drush config:import, then no type is created.

What is the purpose of these collections? Is there some way to indicate that you want collections to be activate/inactive within the configuration? Do they serve some completely different purpose?
Collections are for configuration overrides. Mainly used for translations, but could be used for other purposes as well. You can list the available collections in your Drupal installation, starting with the default collection (empty string):

$collections = \Drupal::service('config.manager')-&gt;getConfigCollectionInfo()-&gt;getCollectionNames();


More info https://www.drupal.org/node/2268523