$\newcommand{\Dev}{\operatorname{Dev}}$ $\newcommand{\dev}{\operatorname{dev}}$ $\newcommand{\grad}{\operatorname{grad}}$ $\newcommand{\divg}{\operatorname{div}}$ $\newcommand{\Ei}{\operatorname{Ei}}$ $\newcommand{\tr}{\operatorname{tr}}$

The SurfacePair section defines a pair of surfaces that can be used to define surface-to-surface interactions (e.g. contact).

<SurfacePair name="contact1">
  <primary>surface1</primary>
  <secondary>surface2</secondary>
</SurfacePair>

The surfaces (i.e. surface1 and surface2) are surfaces defined in the Surface section. Because surfaces must already be defined before they can be referenced in the SurfacePair section, the Surface must be defined before the SurfacePair section.