In about two weeks many free software developers and enthusiast meet at the FOSDEM in Brussels for at least two days. They are traveling from other countries inside the European Union as well as from countries around the world to Brussels. Many developers (and some open source supporters too), which attend the event are employed by open source companies, they are traveling to Brussels within the scope of their payed job (for different reasons) and give talks on topics around their dayly work.
If there is a connection between the the payed job of an attendee and the participation at the FOSDEM, people from inside the European Union that travel (in this case) to Belgium, Brussels need a form A1. If they didn’t own such a certificate their work runs under the rules of the Belgium social security. I explained the features of the A1 form already some weeks ago on my blog here. A connection to the daily could be e.g. a presentation with the branding of the employer or even the statement of a business trip at the hotel reception.