If I understood your question correctly, you were also asking about the different levels. At the national level, in the main committee, the trade unions are represented, but they are also represented at the local level. The trade unions participate at all levels. That goes back to a time in the late 19th century when social laws were developed in Germany under Bismarck. They wanted to make sure that not only the employers participated in these very important social developments, but also the trade unions.
On February 5th, 2013. See this statement in context.