What is the segment addition postulate in geometry and how does it work?

The segment addition postulate states that if point ##B## lies on a line segment ##AC## then ##AB +BC = AC##.
Example:
Prove that XY has a length equal to 6.
##XY + YZ = XZ## (Segment addition postulate)
##XY + 11 = 17## (Substitution of values based on given)
##XY = 6##