The tax on packaged sugary drinks is a tax of the Generalitat de Catalunya, the purpose of which is to tax the consumption of packaged sugary drinks due to the effects they have on the health of the population and its main purpose is to encourage a change in consumption habits, as recommended by the World Health Organization (WHO).

The tax, created by Law 5/2017, of 28 March, on fiscal, administrative, financial and public sector measures (DOGC no. 7340, of 30.03.2017), enters into force on 1 May 2017, in accordance with the provisions of Decree Law 2/2017, of 4 April, amending the entry into force of the tax on packaged sugary drinks and establishing a rule for determining the applicable rate of tax on stays in tourist establishments (DOGC No. 7345 of 6 April).

The law approving the tax came into force on May 1, 2017.

The taxable amount is the quantity in liters of packaged sugary drink delivered by the distributor and purchased by the taxpayer. That means that the quantity to consider to calculate the tax is not placed in the quantity, since taxpayer doesn't buy liters but units.

This tax needs to be posted properly in the invoice.

The idea or suggestion could be summarized as having the ability to calculate the tax based on a different UoM than the one in the sales line, but related as a conversion.
This ability could also fit new tax requirements in the future, when those kinds of taxes will be more and more common.