This is the method I use to create a parte which will get it's final dimensions when it is inserted in an assembly. This example will use a part with holes, the amount of holes is changed with the part size.
create jour part.
create two sets of holes.
Supress one of the sets of holes
Change dimension d0 into Width, this will be our variable dimension. To do this go to the "manage" tab and click on "Parameters"
Then go to the "manage" tab and click on "Add rule" then enter the same text as I did. This will result in:
When the width is less than 75 mm there will be 2 holes in the part, when the width is larger than 75 mm there will be 4 holes in your part.
Click on the little arrow under iLogic and Add form. Name your form.
Grab and pull the variable "Width"to the rigth window and click ok.
Test your Form. You can put as much variables into your rules and forms, but for this tutorial I kept it simple. Just enjoy.