See section 12.5.4 for vectors in space.
The segment and vector commands return and draw vectors. (The segment command can also draw line segments, see section 11.7.3.)
segment takes as arguments a point and a vector. The point indicates the beginning of the result and the vector (given as a list of coordinates) the direction.
segment returns and draws the corresponding vector as a line
segment. If the arguments are P and V, then command
draws the segment from P to P+V.
vector takes as arguments two points (or a list with two points) or a point and a vector.
vector returns and draws the corresponding vector. If the
arguments are two points, the vector goes from the first
to the second point; if the arguments are a point and a vector, then
the vector starts at the given point.