The study of the concept of "environmental tax" included in the GBER and in the guidelines, checked against the notion of "energy tax", demonstrates that in the field of State aid a specific legal configuration is used. The latter stems from the reference to both the taxable base and the beneficial effects "from an environmental point of view" obtained from the tax concerned.
In the opinion of some researchers, this way of articulating the concept of "environmental tax" causes a wide area of legal uncertainty and the exclusion of certain energy taxes and its incentives which fall outside environmental policies. However, other researchers point out that the Energy Taxation Directive covers almost all the relevant energy taxes and thus, the concept of "environmental taxes" together with the characterisation of harmonised energy taxes as environmental would provide enough certainty in this regard. Besides, the environmental protection effect should not be proven but it is construed by law in all relevant cases and so are the transparency and incentive effect.