A 2D or 3D model is created using the following building blocks:
•  Regions: A region may contain either a polygon or circle. Material properties are applied to regions. 
Helpful links: Regions Dialog; Region Properties Dialog; Model Menu  Geometry
•  Polygons: Basic geometric building blocks may include closed polygons. Only contain x and y coordinates. In 3D, polygons extrude vertically through surfaces to define specific model blocks. Only one polygon or circle can be placed in a region. 
•  Circles: Basic geometric building blocks may include circles. Only contain x and y coordinates as well as the circle radius. In 3D, circle extrude vertically through surfaces to define specific model blocks. Only one polygon or circle can be placed in a region. 
•  Surfaces: Basic building blocks of a 3D numerical model. 3D models are defined by a series of stacking surfaces. Each surface may be specified by a grid, an expression, or a constant. Specific modeling "blocks" are defined by the projection of region polygons/circles onto surfaces. Each surface grid may be formed of regular or irregular lines in the X or Y directions. Surface grids in a single model do not have to consist of identical grid intersection points, however, if there are complex pinchouts between surfaces it helps to have matching surface grids as mesh generation errors will be minimized. 
Helpful links: Surfaces Definition
•  Layers: Formed by surfaces on the top and bottom. A layer is the volume between two surfaces. Therefore, a model will contain one fewer layer than it does surfaces. 
