A load backrest is defined as:

"load backrest: that portion of the carriage and forks serving to restrain the load when the load is tilted rearward or upward." (ANSI/ITSDF B56.1-2005, Safety Standard for Low Lift and High Lift Trucks, Part IV, Glossary of Commonly Used Words and Phrases).

"load backrest - The portion of the carriage and forks which supports the load when it is tilted rearward, upward or elevated." (The Raymond Corporation, Glossary).

"(1) The vertical shanks of the forks and/or the front face of the carriage are considered to be the load backrest." (B56.1 Interpretation, I-32, ASME/ANSI B56.1-1988).

In short, the load backrest is formed by the carriage frame and the fork uprights (shanks).
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A load backrest extension is defined as:

"load backrest extension: a removable device that increases the load restraining area beyond that provided by the load backrest. (ANSI/ITSDF B56.1-2005, Safety Standard for Low Lift and High Lift Trucks, Part IV, Glossary of Commonly Used Words and Phrases.)

"load backrest extension - A device extending vertically from the fork carriage frame (load backrest)." (The Raymond Corporation, Glossary).

In short, a load backrest extension is any vertical extension used to increase the load restraining size beyond that of the carriage and fork uprights (shanks).
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The load backrest extension can come in various sizes and is usually bolted to the carriage so it can be removed. In some instances it may be welded to the carriage.

Some load backrest extensions also serve as end stops for the fork carriageway. A forklift should not be used without end stops on the fork carriageway as the fork locking device may be defective, fail, or be improperly set and a fork could vibrate or jar loose. An operator could also fail to notice a lack of an end stop and slide a fork off while making fork adjustments.

Contact the forklift manufacturer for written instructions on temporary end stops when a load backrest extension which also functions as a fork carriageway end stop is to be removed for certain loading activities.
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