Logic behind Transaction Nature for Brazil

Hi,

In p4310BR and P4210BR the system calculates the Transaction Nature 3 digit, X.YY , the X digit is clear how is calculated, but the second digit documentation only says ;

This digit is an algorithm calculated by the system, not setup anywhere; it will depend on the type of transaction that is being done.

1 - Sales or Purchase

2 - Transfer item

3 - Return

7 - Sales/Purchase with tax substitute

9 - Other...

(Note: This number will not allow the system to calculate ICMS and IPI recoverability or ICMS substitution and reduction)

my customer is using 9, but we want to know how the system will decide which one applies, I know the whole BNOP can be default it from Processing option, but also calculated.