button in the simulation toolbar allows you to limit the portion of the part that is rendered during a 3D solid simulation 
or rapidcut 
simulation. There are two primary reasons to do this:The region of interest button in the simulation toolbar allows you to limit the portion of the part that is rendered during a 3D solid simulation or rapidcut simulation. There are two primary reasons to do this:
The rest of the part is not simulated and it is therefore faster to simulate your region.
If you are using rapidcut, the region is simulated at a finer resolution.
There are three different types of regions
Stock: This is the default type of simulation where the entire stock is rendered during simulation
Feature: Select the name of the feature in the drop down list and a region around the feature will be selected as the region.
XYZ Location: The region is defined by either a box you drag or by two points whose coordinates you can enter. Note that the sides of the box are aligned with the X and Y axes.
See also Simulation types, Using_simulation_VCR_controls or Generating/simulating toolpaths