Does the Scarlett 18i6 have "loopback"?
No, the 18i6 doesn't have the loopback function due to a limitation of DSP. However there is a workaround for this, using the SPDIF in/out. If you loop a cable from the SPDIF output to the SPDIF...
Changes to User Manual
In the manual, you will find the following feature:
Sample Rate Changing
Unlock – Allows any WDM or ASIO application to change the Sample Rate.
ASIO only – Allows only ASIO applications...