When you put a shell mesh on a face, the mesh has the concept of a Top surface of the element a distance t/2 away from the face and a Bottom surface of the element a distance of t/2 on the other side of the face.  When frictional contact is defined to a shell mesh, you tell it if the contact is with the Bottom or the Top surface of the element.  Pressure is a bit different. It is applied to the face, and it shows with an arrow the direction the pressure will be applied. If you don't want that direction, put a negative sign on the value.