To add to what Peter said, Contact elements are constrained against penetrating the target surface. However, target elements can penetrate through the contact surface. For rigid-to-flexible contact, the designation is obvious: the target surface is always the rigid surface and the contact surface is always the deformable surface. For flexible-to-flexible contact, the choice of which surface is designated contact or target can cause a different amount of penetration and thus affect the solution accuracy. Consider the following guidelines when designating the surfaces:
1. If a convex surface is expected to come into contact with a flat or concave surface, the flat/concave surface should be the target surface.
2. If one surface has a fine surface mesh and, in comparison, the other has a coarse mesh, the fine mesh should be the contact surface and the coarse mesh should be the target surface.
3. If one surface is stiffer than the other, the softer surface should be the contact surface and the stiffer surface should be the target surface.
4. If higher-order elements underly one of the external surfaces and lower-order elements underly the other surface, the surface with the underlying higher-order elements should be the contact surface and the other surface should be the target. However, for 3-D node-to-surface contact, the lower-order elements should be the contact surface. The higher-order elements should be the target surface.
5. If one surface is markedly larger than the other surface, such as in the instance where one surface surrounds the other surface, the larger surface should be the target surface.
6. In the case of 3-D internal beam-to-beam contact modeled by CONTA176 (a beam or pipe sliding inside another hollow beam or pipe), the inner beam should be considered the contact surface and the outer beam should be the target surface. However, when the inner beam is much stiffer than the outer beam, the inner beam can be the target surface.
These guidelines are true for asymmetric contact. Asymmetric contact is defined as having all contact elements on one surface and all target elements on the other surface. This is sometimes called "one-pass contact." This is usually the most efficient way to model surface-to-surface contact. However, under some circumstances, asymmetric contact does not perform satisfactorily. In such cases, you can designate each surface to be both a target and a contact surface. You can then generate two sets of contact pairs between the contacting surfaces (or just one contact pair; for example, a self-contact case). This is known as a symmetric contact (or "two-pass contact"). Obviously, symmetric contact is less efficient than asymmetric contact. However, many analyses will require its use (typically to reduce penetration).