Leupold RX Operating Instructions Manual page 19

BAS will display the equivalent horizontal range.
This is the range that you will want to shoot at,
instead of the line of sight distance, which can be
grossly off depending upon the angle of the shot.
The display will read the equivalent horizontal range
in the measurement you have chosen previously.
NOTE: True Ballistic Range is available only in the RX-II,
RX-III, and RX-IV models.
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