A line is a series of points strung together to form a chain.
The simplest form of line geometry is a two-point line: a line with a start coordinate and an end coordinate, but no intermediate points.
A polyline feature is a multi-point line; that is, it has a start coordinate and an end coordinate, plus one or more intermediate points.
An NB is a polyline feature whose last point is coincident to its first. In other words, it appears as a closed figure. However, it is a line feature by definition and doesn't become a polygon unless coerced (connected) by a reader/writer or transformer, such as FME Workbench's PolygonBuilder.