This is how I understand it…
- Relative offset is defined by the bounding box of the object. So a factor of 1 = the size of the bounding box.
- Constant Offset is calculated from the center of the object. The factor is measured in whatever unit of measurement you’re using (meters, feet, etc)
I wish I had good use case examples for you. I use Relative Offset when I am blocking out a character and determining proportions. I can put an array modifier on the head in the Z axis to determine the head-height of the character. I haven’t had reason to use Constant Offset that I can recall so maybe someone can pop in with a good example?