Tax return: how do I fill in box 2042 RICI?

The 2042 RICI is indeed for rental income, but heads up: it depends on your tax regime. If you're on the actual expenses regime (you declare charges, repairs, loan interest, etc.), you need to use the full form with all the schedules. If you're on the micro-rental regime, you just report the gross income and the tax authority applies the standard deduction, so it's simpler but less advantageous if you have a lot of expenses.

Problem: you mention a property in Liège and one in Brussels. Liège is Belgium, not France. If it's really in Belgium, you need to declare it to the Belgian authorities, not the French tax administration. The 2042 RICI only covers rental income in France. Same thing for the apartment in Brussels - that's Belgium too. Double-check where your properties are located before you fill anything out, because if you have Belgian income and you're a French tax resident, the reporting rules are completely different and there are often agreements in place to avoid double taxation.

If one of your properties is actually in France, first look at what category it falls into (furnished or unfurnished, long-term or short-term rental), then contact an accountant or your local tax center to confirm whether the actual expenses regime or micro-rental regime works better for you. Honestly, with the complexity of cross-border income, you don't want to mess up which form you're using.