Dimension Selection Dialog Box
If sketched beam sections are present in your model, clicking 
in the design study Variables table opens the Dimension Selection dialog box. Use this dialog box to select the type of dimension and to add the dimension from the model to the design study. This dimension is added as a variable.
in the design study Variables table opens the Dimension Selection dialog box. Use this dialog box to select the type of dimension and to add the dimension from the model to the design study. This dimension is added as a variable.You can select the following types of dimensions:
• Model Dimension—This option is selected by default. Click OK to select and add the dimensions in your model as variables. You cannot select beam sections in the model if you select this option.
• Beam Section Dimension—Use this option to select the listed Beam Sections. Click OK to select the beam section dimensions of your model from the sketcher to add as design variables.