The tax exemption reason of the line, as reporting to the tax authority needs it on a zero-rated or exempt line: a code the tax rate of the line offers (the TaxExemptionReasonCodes page lists them), or, on the KSA out-of-scope rate, the reason written out. On a rate that takes no reason, a reason sent is kept but not reported. null or "" clears the reason. Left out, a new line takes the default of its item, else the default of the contact (when the line is on the default revenue tax rate of the contact), else the default of its tax rate, else none. So does a line whose tax rate or item changes. Any other line keeps its reason, also when an update sends the lines again: each line is compared with the stored line at the same position.
