When doing VFX and tracking, there is an option "Set Scale" to set the relatively same scale to the 3D interface as the video you're tracking. What is the actual size for the scale (is the scale in ft, in, mm, m? etc.)
The default scale is blender units...(1m per blender unit), but if you are using different units for the scene then the units will be whatever you want.
The purpose of that option is for you to match known measurements. If you are tracking objects or markers and you know the distance between them, you can use that measurement (along with the lens and sensor size) to make the 3D reconstruction more accurate and closer to the original scene you are tracking.