When joining a ReadyNAS OS 6 device to an Active Directory domain, there is an option called "Do not cache ADS accounts locally" (marked in red below).

What does this option do?

When joining a ReadyNAS OS 6 device to a domain, disabling this option (the option box is clear) means that the ReadyNAS imports a list of users and groups from the Active Directory server. If the option is enabled (the option box is checked), then the ReadyNAS does not import the list of users and groups.

When joining a ReadyNAS OS 6 device to a domain, should this option be enabled?

If there is a very large number of users and/or groups in the Active Directory server (for example more than 20,000), then importing the full list of users and groups might slow down the domain join process and prevent the ReadyNAS GUI from performing optimally. In this situation, you should enable the "Do not cache ADS accounts locally" option before joining the ReadyNAS to the domain.

What are the implications of enabling this option?

As explained above, enabling this option when joining to a domain means that the ReadyNAS does not import a list of users and groups from Active Directory. Authentication via Active Directory still works seamlessly, however. Because a list of users and groups is not cached in the ReadyNAS, it is not possible to set permissions on shares using the ReadyNAS GUI. You can still set permissions directly on the shares and folders using Windows.