For 2016 or newer models (or seats retrofitted with update rear-facing lock-offs):
The vehicle belt or LATCH (UAS) Belt MUST be routed and secured under both rear-facing lock-offs for all rear-facing installation.
If the vehicle belt has a plastic button or other feature which prevents the use of one of the lock-offs, then you may install the seat using only one lock-off. Leave the other lock-off closed and route the vehicle belt or LATCH (UAS) Belt over top.
To check if your vehicle is equipped with an ALR system, pull the seat belt all the way out from the shoulder retractor. Slowly release the belt back into the retractor, and if it locks when you attempt to pull again, then the ALR has been engaged. Please refer to your vehicle owner’s manual to see what type of seat belt system your vehicle has.