The system is called "excess profit ruling" and is perfectly legal. It means that a Belgian branch of a multinational only has to pay taxes on the profit it would make as a single company, without the benefits of its international network. The profits generated by the fact that it is a multinational that has already built a solid reputation abroad, the so-called "excess profit" is being ignored by the Belgian taxman. Companies do have to negotiate this with a ruling commission first.
While the system may be legal, experts have voiced criticism also taking into account the recent news about Lux Leaks. The profits generated by multinationals in Belgium, are not being taxed automatically elsewhere either (like in the home country where the business is based). This is because Belgium is not informing foreign administrations about the rulings.
In fact, the Federal Economy Department has been promoting the Belgian system for years. The fiscal expert Michel Maus says that the rulings "may go further than Lux Leaks. (...) We don't know how many rulings have been made, and how many of these presents have been given (by the taxman." The Lux Leaks revealed the generous tax deals Luxembourg offered to wealthy people, allowing them to evade taxes in their home country.