Reasoned opinion

Any national Parliament or any chamber of a national Parliament may, within eight weeks from the date of transmission of a draft legislative act, in the official languages of the Union, send to the Presidents of the European Parliament¹, the Council² and the Commission³ a reasoned opinion.

This should state why it considers that the draft in question does not comply with the principle of subsidiarity. It will be for each national Parliament or each chamber of a national Parliament to consult, where appropriate, regional parliaments with legislative powers.

Consolidated version of the Treaty on the Functioning of the European Union, Protocol No 2 on the application of the principles of subsidiarity and proportionality, Article 6

1.
The European Parliament is the EU's law-making body. It is directly elected by EU voters every 5 years.
What does the Parliament do?
The Parliament has 3 main roles:
Legislative
2.
In the Council, government ministers from each EU country meet to discuss, amend and adopt laws, and coordinate policies. The ministers have the authority to commit their governments to the actions agreed on in the meetings.
Together with the European Parliament , the Council is the main decision-making body of the EU.
Not to be confused with:
3.
The European Commission is the executive body of the EU and runs its day-to-day business. It is made up of the College of Commissioners, 27 European Commissioners, one for each member state, who are each responsible for one or several policy areas. In addition, the 'Commission' also refers to the entire administrative body that supports the Commissioners, consisting of the Directorates-General and the Services.