Yes, you have to set the processing option for R09801 to update F0018 with the tax information of the transactions you are posting. In 8.9 (earliest documentation I have), it is processing option number 1 on the Taxes tab. You must enter a value of 2 or 3 to also post tax explanation code "S". (A value of 1 only posts the tax explanation codes which start with "U" and "V".) The difference between a value of 2 and a value of 3 is the treatment of tax explanation codes starting with "E". These are the tax exempt tax explanation codes. If you want them in F0018 for audit trail purposes, you must use a value of 3.
Make certain that all versions of R09801 that are used to post vouchers have the processing option set. Delivered versions ZJDE0002 and ZJDE0005 post vouchers. You also need to set the processing option on any custom versions you have created.
The bad news is, this affects new postings only. I know of no programmatic way to get the tax information from already posted vouchers into F0018. The tax information from already posted vouchers can be entered manually using P0018. You could write a custom UBE or use UTB to identify the missing items.
