Extension Bar is a coupler interposed between the socket and the driving tool. This is used to reach in accessible places where the driving tool cannot reach or the clearance for operation of driving tool is not available.
These are of 2 designs:
Square drive adaptor with 2 cross holes for conventional impact wrench.
Square drive adaptor with groove for locking ring and pin for impact wrench with cross hole in male square drive
The range covers 1/4" (6.3 mm) to 2.1/2 " (63 mm) square drives in different lengths.