Remachining operation is useful for pocket machining.
For example , you need to create a triangular pocket like in the image below.
For the roughing operation you choose an end mill with a certain diameter.
Roughly speaking , bigger is diameter of the endmill, faster is the operation machining time. (This statement is very imprecise , but it help understand better the example ).
But in the inner corner you get more material left to remove.
To reduce the corner material , you can enable the RE-MACHINING operation.
You need to select a tool with smaller diameter for this operation.
Below the generated toolpath.
You can see the material in the corner is reduced and the machined area from roughing operation is skipped. Only the corner area is machined.
By default, the toolpath engine will recognize the diameter used in roughing operation, but is possible set a different "Previous Tool Diameter" enabling the manual mode and setting the required value.