An insert is another mold component that is typically used as a cost saving measure. The mold uses the same core and cavity, but one insert is swapped for another. Different inserts can be used to create different shapes. Thus, you can use the same mold to create similar parts simply by switching inserts. In the upper-right figure, an insert needs to be created for a square cut in the bottom inset of the reference model. The resulting insert mold volume is shown in the lower-left figure. However, the model could have a design variation where, rather than a square cut in the bottom, there is a round cut in the bottom, as shown in the lower-right figure. In this case, you can create a different insert mold volume, while you use the same core and cavity.
You can also use inserts in areas that are difficult to machine.
Because mold components are ultimately created from mold volumes, you can use sketch-based features to create insert mold volumes in the mold model.