I am not sure I am familiar with the concept of an AF Element Type... are you referring to the AFReferenceType argument in a few of the AFElements.Add method overloads? If that's the case, then the AF Reference Type is what specifies the relationship between the 2 elements. You can create additional Reference Types in the PI System Explorer, under Library (you can also create them programatically using the AF SDK, but there is probably less of a use case for this).
I suggest you give a look to the "Defining Reference Types" in the AF User Guide (available on the vCampus Library, under the vCampus PI Products Kit > Server Products branch). Should you need more information on reference types and configuring AF Elements in general, I invite you to contact our regular Technical Support team. For programming questions, this forum is the right place!