[jdiwnab]

Inventor is saying that you can't dimention some part that is already made. You are probably making a sketch on a face of an already extruded part. That face's geometry is now part of the sketch, but you can't change the dimentions of it in you current sketch. If the part is the wrong size, you need to go to it's sketch that you made it from to change it.

One thing you have to remember is that Inventor is what they call Parametric Modeling. This is a fancy term to say that every dimention is related to the others. If you make an extrution and then make another extrution the same shape as the first, then edit the first one, the second one will change. That isn't the only example, but is easy to understand. Also, if you want an object to be always square, no mater what size, you can tell one dimention on a rectangle to be whatever the other one is.

When you dimention something in a sketch, you are really telling it the size it needs to be. AutoCAD tells you how long something is. Inventor tells you how long something is and wants you to change it. To do the same thing in AutoCAD, you have to remake a part.

The power come from, aparently, a huge database of dimentions and other things you have made. Our chassis model is in an assembly and has about 600 dimentions, including constraints, which are dimentions of a different type.