The range of a single target measures its distance from the center of the transducer face. The beam-axis distance is a related concept which measures the distance of a single target from the transducer face along the beam axis.
Beam-axis distance is visible only on the 2D projections graphs and ensures that a straight line in physical space (as a fish may swim) appears as a straight line on the projections displayed on this graph. It is orthogonal to both the minor-axis and major-axis distance (which measure distance along the minor and major axis of the transducer respectively).
The difference is illustrated below.
Note: Beam-axis distance cannot be exported, and is not used in the calculation of range related analysis variables.
About depth and range mode
About single target depths
About analysis variables