Electromagnetic and Radio frequency interference (EMI/RFI) shield design is complex and multi-faceted, combining electrical and mechanical engineering concepts. In practice, shielding is very much an iterative process in which controlling one variable impacts another, leading to even more changes in the design.
One aspect of shield design is knowing which external signals are most critical to keep from interfering with the circuits inside. This is important from the standpoint of material selection, both for the shield structure and any subsequent plating. Knowing the frequency and amplitude of critical signals and how they behave with conductive metals helps you determine the conductivity and thickness required to achieve adequate shielding effectiveness.