A segment is a part of of the contour between two neighboring nodes.
Segments between two sharp (corner) nodes are called line segments.
Segments between sharp and smooth or between two smooth nodes and are called curve segments.
Curve segments can be of two types: PostScript (PS, cubic Bézier) curves and TrueType (TT, quadratic) curves. Different types of segments are prohibited in one contour in final font formats but are allowed in FontLab VI.