When performing design studies in SolidWorks, you can typically link the dimensions within the model to design study parameters and then vary those dimensions to undertake design studies or design optimizations.
If you use shell elements in an analysis then the thickness of the shell is defined within the simulation setup and is not part of the model itself. To perform a design study on the thickness of the shell, the thickness needs to be linked to a simulation parameter. To define this parameter, the Link Value option within the shell definition property manager can be used. Once the link value option is selected, a Parameters dialog box pops up. This can then be used to define a “Simulation Parameter” and linking this parameter to the thickness definition of the shell. Once the parameter is appropriately linked, it appears in the parameters list in the design study variable view window.