TopAbs_Orientation

There is a section describing orientation in
the OCC Modeling Data User's Guide. For the
version shipped with OCC 5.1, this is on page
27 (the 29th page). It describes these
orientations and shows a picture.

from the source code::

//! Identifies the orientation of a topological shape.
//! Orientation can represent a relation between two
//! entities, or it can apply to a shape in its own right.
//! When used to describe a relation between two
//! shapes, orientation allows you to use the underlying
//! entity in either direction. For example on a curve
//! which is oriented FORWARD (say from left to right)
//! you can have both a FORWARD and a REVERSED
//! edge. The FORWARD edge will be oriented from
//! left to right, and the REVERSED edge from right to
//! left. In this way, you share the underlying entity. In
//! other words, two faces of a cube can share an
//! edge, and can also be used to build compound shapes.
//! For each case in which an element is used as the
//! boundary of a geometric domain of a higher
//! dimension, this element defines two local regions of
//! which one is arbitrarily considered as the default
//! region. A change in orientation implies a switch of
//! default region. This allows you to apply changes of
//! orientation to the shape as a whole.