VMware Cloud Director 10.x
The "Automatically download the content from an external catalog" flag simply indicates whether to synchronize the VMDKs when a synchronize event is initiated. If it is unchecked, it will simply populate a 'skeleton' of the remote items when you synchronize and you will have to sync items individually. In the meantime, the status will be CONTENT NOT AVAILABLE. If it is checked, you do still need to initiate a synchronization by clicking Sync, but it will pull the VMDKs for each of the items.
If the 'Automatically download the content from an external catalog' is turned off, manually synchronize the individual items that are required. If the flag is enabled, either synchronize the individual items or simply initiate a synchronization from the subscribed catalog.