Lock-offs are built-in features on some child restraint system (CRS) which lock seat belts and secure the CRS in a similar method as locking clips (click here to read more on locking clips.)
They are designed to clamp the shoulder portion of the seat belt, creating a fixed length of webbing and hold the vehicle seat belt tight to maintain a good installation throughout.
There are many types of built-in lock-offs, as can be seen in the diagram. Lock-offs come in many different styles and can be very obvious color-coded levers and latches. Many CRS have lock-offs on both sides so that they can be used for installation on either side of the outboard locations. Other CRS has just one lock-off at the center. There is a possibility of different lock-off used for different orientation.
It’s easy to get confused, so be sure you read the CRS manual to find out which lock-off you’re supposed to use.