What is the difference between measurment range and indication range in ELISA re - (Jan/03/2013 )

My ELISA reader specs measurment range =3.0 OD and indication range =3.5 OD,what is the indication range and what is the difference?

I suspect that the number is linear with colour intensity over the range 0 to 3, and less accurate between 3 and 3.5. In other words, don't go above 3 units for quantitative assays