[IBIS]: Value of USE_NA and NOT_USED

Hi All

I was curious to know why the values used in the S2IBIS3 source code for 
USE_NA and NOT_USED are as shown below.

*USE_NA            =   -1.33287736222333E20;
NOT_USED      =   -1.33287736222333E18;*

Is there any purpose or just is it fine to use any other values.

Thanking you
Shrikant GK


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