Include() is great for running code within another JSL script. You should also look into defining your own functions, because then you can call them within your larger script with parameters. User-defined functions are documented in Help > Books > Scripting Guide, on page 258 (version 12.2).
Here's a simple example function:
add3 = Function({a, b, c},
// Add up the three arguments
tmp = a + b + c;
// The last statement will return the value of tmp to the caller
tmp;
);
Here are some sample calls to this newly defined function:
x = add3(1, 5, 9);
z = add3(3.14159, 2.7183, 6.721);