In the sales order header, on the tab "Billing Document", there exists a possibiltity to influence the tax determination. In the block "Billing" fields exists to enter manually an alternative tax classification for the customer:
The fields are named VBAK-TAXK1, VBAK-TAXK2 until VBAK-TAXK5.
Those fields are never filled, see thread TAXK1 through 3 not populating into Sales Order
Instead the intention is to override in a specific sales order the usual tax classification from customer master, for this one document only.
Which field should be filled depends on the fields used in the access sequence:
If the tax condition record is found with an access containing field KOMK-TAXK1, then for a manual override of the customer master the first field VBAK-TAXK1 would have to be filled manually.
First the information is taken from the customer master:
Depending on the customizing settings, in this case the tax classification of the ship-to-party is used.
Only when field VBAK-TAXK1 would be filled, then this value would be overwritten here:
Here VBAK-TAXK1 would replace the customer master data, when it would have been entered in the sales order header.
VBAK-TAXK2 would be taken into TKOMK-TAXK2. On the tax determination this would have only an effect, if in any access sequence of the tax condition the field KOMK-TAXK2 would be in use.