Why is the line between frictional and structural unemployment sometimes hard to draw?

What will be an ideal response?

Suppose a highly skilled steel worker is laid off because his company shuts down its plant in his area and moves to a new location overseas. The worker would like to find a comparable job, but only low-wage, unskilled work is available in his town. Jobs are available, but not his kind of job; and the steel company will not come back. There is really no correct answer as to whether this is frictional or structural unemployment.