When assembled, I have parts that need planes parallel to my assembly planes. This will allow me to make normal cuts to a plane, and "hopefully" allow me to cut on curves and angles to mate up well with other parts within the assembly. I can measure, offset, angle, and whatever to try and make a plane on my part that is parallel to my assembly, but it's rather tedious, and not always working out so well. Is there an "easier" way? I'm using ST4, if that matters.
When placing a plane in a part that you are editing-in-place from an assembly, hold down the SHIFT key to allow selection of assembly planes and other part faces as references. This should get you what you want!