In an in-place interface there is no physical separation in space between the two boundaries comprising the interface. Furthermore, there is no requirement that the two boundaries have the same perimeter. After the intersection occurs the original boundaries maintain their boundary type and retain whatever surface area remains (if any).
The figure below shows different examples of in-place interfaces. The shaded red, green and blue areas represent different mesh regions, such as porous/non-porous or fluid/solid combinations. The dotted black line in each case would represent the in-place interface that would be created between the coincident boundaries for each region. Where the areas of the original boundaries differ then remainder boundaries are produced.