If you have a steel frame, and steel fork. Then the BB drop will be a measurement from the dropouts to the center of the bottom bracket. And should be fixed for that frame, and often is fixed for the model.

https://surlybikes.com/bikes/ogre



If you had suspension (front or rear), then the droputs would move, and the drop should describe a normal range. At least Trek seems to list "Geometry Position" "High/Low" which seems to take into account the suspension on their full suspension frames.