There is an integer linear programming problem with this constraint:
$$\left|X1 - X2\right|= 5\text{ or }10\text{ or }20$$
How it can be solved with adding auxiliary variable $y$?
Main problem is that "or" between $5$ and $10$ and $20$.
Here is a tactic using 3 auxiliaries:
$$|X_1 - X_2| = 5y_1 + 10y_2 + 20y_3$$
$$y_1 + y_2 + y_3 = 1, all binary$$
Now you just need to model the left-hand-side, which isn't trivial.