Analytical iTwin and RAM Concept model slab areas differently. It is instructive to describe the differences in detail here to explain how the import and export operations are affected.

RAM Concept slabs are defined by a collection of slab areas and openings with arbitrary overlapping polygonal boundaries. Each slab area defines material, thickness and surface elevation properties. An integer priority determines which slab area or opening takes precedence where two or more slab areas overlap.

Analytical iTwin slabs are defined by a collection of surface members with polygonal boundaries. Each surface member may contain any number of surface member modifiers. The surface member and its modifiers define the slab material, thickness and surface position properties. Modifier boundaries must lie inside the parent surface member's boundary. Modifier boundaries may overlap, so modifiers have an integer priority to determine precedence in overlapping areas. Modifiers always take precedence over the parent surface member. Normal practice is for modifier priorities to be sequential, starting at 1.

A surface member may also contain any number of surface member openings. Like modifier boundaries, opening boundaries must lie within the parent surface member's boundary and may overlap. However, openings always take precedence over the surface member and its modifiers. In effect, surface members have an infinitely low priority, surface member modifiers have an explicit integer priority, and openings have an infinitely high priority.

Therefore, Analytical iTwin surface member boundaries may overlap, as long as there is no overlap between the surface member effective shapes.