British Airways plc v Sally Williams and others  UKSC 16, the Supreme Court of the U.K. has made its first "preliminary reference" to the Court of Justice of the E.U. The case concerns the remuneration of pilots working for British Airways. It is necessary for the Supreme Court to determine the meaning of certain "Directives" before the court can finalise the case and, since there is no further national appeal, the Supreme Court is obliged to make the reference unless the meaning of the EU legislation is "clear" (the so-called "acte clair" doctrine). There have been many references to the Court of the Justice of the EU which, until the Lisbon Treaty came into force, was known as the European Court of Justice (ECJ). Among the most famous references were those in the Factortame litigation when the ECJ asserted the supremacy of EU law over even an Act of Parliament. [The "Supremacy of EU Law" was established by the ECJ very early in the history of the original "common market" and the British government's stance in Factortame was doomed to fail from the outset].
The preliminary reference procedure is set out in the Treaty on the Functioning of the European Union and is explained here. Without such a procedure, national courts would be entirely free to interpret things in their own way and there would not be a uniform interpretation within the EU of the Treaties or other legislation.
On a rather lighter note, I was interested to see a case mentioned on the European Law blog which concerned taxation of places where people sit in their own individual booths watching porn movies! Have a look at the blog - "Dirty Movies, Solitary Pleasure and Concept of Cinema".