Question

What is the subsidiarity principle? Is it relevant for all EU legislation?

Answer 1

It is the principle that the EU will not take action (save in areas where it has exclusive jurisdiction) unless it is more effective than national, regional, or local action.

Yes it is relevant since it aims to protect the ability of Member States to make decisions and take action by authorizing intervention by the Union where the goals of an action cannot be met adequately by the Member States but can be achieved more effectively by the Union.