The TAX CODE field is the tax code that will be used on the invoice for any sales tax. If you do not select a TAX CODE to use, we will create a default tax code, and tax agency with the name: "[POS Type] Tax". So, if you are using a clover point of sale, the default tax code name would be "Clover Tax".
This field is only used in your accounting system if you are not using the quickbooks online automated sales tax. If you are using the automated sales tax, then quickbooks will choose your tax code automatically.