Mandatory disclosure rules (MDR) are a new tax institution in EU. There were introduced by Directive 2018/822 in 2018 and have already been implemented by some EU countries. The directive imposes an obligation to report a cross-border arrangement (that could concern either an arrangement between more than one European Union country or between EU country and other country), that meets the features listed in the directive.
Poland is the first country that implemented these rules (in wider approach that are required by the directive), lack of applying MDR in daily business operations could result in being penalised and subject to administrative proceedings, finally employees of a company may be punished under the Criminal Code or an administrative fine (even 10 000 000 PLN).
In my opinion this new tax institution should be verified as part of the due diligence processor an acquired company and obligation to report M&A transaction as tax arrangement.