In Orchestrated software, it can be set up where the tax code on sales and purchasing documents will auto populate so it doesn't have to be picked every time.
On sales documents, the tax code is pulled from the ship to address on the business partner master data.
In order to have the tax code automatically populate as EX, go to Business Partner > Business Partner Master Data, pull up the vendor in question, then select the addresses tab. Inside the address tab, click on the Ship To address that pertains to this tax code and set the tax code from the list that pulls up. In this example, we are using "EX" for the tax code on this ship to address (see highlighted below):
Once finished, click the Update button to confirm. From now on when you create a sales document for that customer using that ship to address the EX tax code will come onto the document automatically.
On the purchasing side, the tax code on an AP Item Type Invoice is pulled from the tax code field on the default warehouse of each line item. On an AP Service Type Invoice however, the tax code defaults to the tax code found on the Accounting Data tab in the Company Details window (go to Administration > System Initialization > Company Details).