Distance dialog box
The Insert | Dimension | Distance menu option calculates the distance between two features. You can select a third feature or an axis for the direction to use in the calculation.
Calculating distance is a bit more difficult to visualize than most of the other dimension calculations. The distance that is calculated between two features uses either the default or straight line method. Two-dimensional distances using a straight line will be parallel to the workplane. Three-dimensional distances using a straight line will be from centroid to centroid.
More:
To Dimension a Distance Using the DISTANCE option:
Tolerances for Distance Dimensions
General Rules for 2D and 3D Distance Dimensions
Relationship for Distance Dimensions