Click the Front Facing check box on the Mesh Operator dialog box if you only want to select the triangles that are front facing in the selection view. When you disable this check box, PC-DMIS selects the triangles that are in both the front and rear facing views.
Enable the Show Graphic Window as Solid option on the Graphic Items toolbar (View | Toolbars | Graphic Items) to see the front and back faces as contrasting colors.
Examples of Selection View (left), Selection with Front Facing disabled (middle), Selection with Front Facing enabled (right)