So, let's get started.
The first program we are going to look at is Inventor; and we are going to look at a part that I have modeled and then we will create a solid to represent a stock solid.
First, I have my model.
Now what I want to do is create a sketch on the XY plane; so, to do this i will go to my model browser and right click on the xy plane and select "New Sketch".
So, now it will start the sketch on the XY plane, and I am going to create a two point center rectangle and snap the center to the 0,0 point of the part. Then i will place fillets on all 4 with a 3/4" fillet.
In the pic below you can see the sketch that i created.
So, once I have this sketch I can now create an extrusion feature.
Now, in this feature you will want to be sure to specify an Asymmetric extrusion because we are extruding a distance above the part and a distance below. Keep in mind this may not always be the case, but in this scenario that is what we are doing because I am going to place the lower portion in a vise to mill on.
Then lastly you want to click on the "New Solid" button which is below the join, cut, and intersect selections.
Once you have your extrusion distances defined just click okay and there you have a second solid body to use as your stock selection.
Hopefully this is helpful and remember this can be utilized whenever you are milling a part that has stock that may have gone through a waterjet process or some other cutting process prior to machining.