GML Simple Features Profile
In both GML Simple Features and previous Simple Features (SF) specifications for OGC, such as Simple Features for SQL, "features are considered to be objects which can have geometry and other properties. The SF specifications are more restrictive than GML, however, in that geometry is limited to points, lines, and polygons (and collections of these), with linear interpolation between vertices of lines, and planar (flat) surfaces within polygons.
The new GML Simple Features (GML-SF) profile has a similar understanding of the structure and geometry of features as SF-SQL. However, GML-SF goes beyond SF-SQL in some important ways: GML-SF supports three-dimensional coordinates (location and elevation) on feature geometry, where SF-SQL just supports two dimensions (location). GML-SF also goes beyond SF-SQL by supporting metadata, a means of referencing local or remote resources which could be used for primary/foreign key references, and dynamic codelists.