When computed class concept is used for auto calculation, it should be a non editable field. But when range is specified to the concept it appears as an editable field in the observation form.
Ideally computed filed should be non editable field. As it appears as an editable field it is creating confusion and giving an impression as a numeric field
Solution 1: Hiding the editable field and only showing the range
Solution 2 : Disabling the field so that no manual entry can be performed
for reference : Appearance on Form 1 forms after having 2 different computed concepts one with range and one without range