The DP claim applies for all crafts
Difference in pay is used and or claimed when you work on an assignment other that yours.
Examples:
1. You are called from your conductors spot and work as a engineer. Your conductor spot
makes more that the spot you worked, you are entitled to the difference in pay
from what you actually made and what your would have made.
2. You work a yard job from your pool spot and your pool spot leaves town with an extra man.
Your are entitled the difference in pay from what you made on the yard job and
what your spot made for the round trip.