Field help (VACITM1) 

This field indicates the tax level applied to the selected product.
The tax level, when crossed with the document tax rule, is used to determine the tax rate to be applied on the document line.

There are several tax levels for a document line:

The tax levels for the document line will be loaded by default with the values associated with the product record ("Accounting" tab).
These values can be modified depending on the transaction.
Provided the user has the corresponding authorizations, he can access the tax level management.