We are a customer with a Group of Companies under a Special VAT Regime (REGE) whereby when 2 of our companies that are eligible for such regime make an intercompany purchase-sale operation, we need to calculate VAT at 0, 8, 10 or 21 percent on a 2nd Taxable Base field that does not correspond to the "Line Amount" of the document line.

In addition, in the printing formats of Invoices and Sales Fees, both Tax Bases must be included together with the calculation of Final VAT applicable to the document.

Imagine a Line of Sale or Purchase with:

Quantity = 2
Unit Price / Cost = 100€
VAT Base s / Art. 78 and 79 = 200€
VAT Base s / Art. 163 = 80€ (new manual entry field -> this is the applicable VAT for the line and could be equals to 1st VAT Base by default)
Applicable VAT %= 21%
Total Amount VAT Included = (2 * 100) + (80 * 0.21) = 216.80€

This double Taxable Base should be displayed throughout the system (Documents Statistics, VAT Books, Invoice and Credit Memo printing Formats, etc.)

I do not know if in other countries there are situations similar to this in which it is necessary to disassociate the calculation of VAT from its "natural" Calculation Base.

In some of our companies this type of intercompany operations are 80% of the total, so it is essential for us that this functionality was implemented in the Business Central standard code since otherwise we are forced to make a huge number of Custom changes on every upgrade.

More Info: https://www.agenciatributaria.es/AEAT.internet/Inicio/_Segmentos_/Empresas_y_profesionales/Empresas/IVA/Regimenes_de_tributacion/Regimenes_especiales/Regimen_especial_del_grupo_de_entidades/Obligaciones_de_facturacion.shtml