score:5
Zaventem is the name of the town where the airport is located, which is why it's often called Zaventem airport. Its official name is now Bruxelles-National/Brussel-Nationaal but this is the same airport. “Brussel-Nationaal-Luchthaven” (literally “Brussels-National-Airport”) is the train station designed to serve it.
Another potential source of confusion is the “Zaventem” train station (not really a station but more a stop with no services now). It's another station in the same town, not very far obviously, but it does not serve the airport (and in fact receives very few trains).
Unlike long-distance high-speed trains in countries like France or Spain, most trains in Belgium indeed have few restrictions, you just buy a ticket and use it on any train like a metro/tramway ticket. As long as it covers your route (including the Diabolo fee where applicable), it's OK. Reserving a seat or getting a discount because you're willing to commit to a specific train isn't even an option, like it might be in Germany for example.
Upvote:3
The "Zaventem airport' station is in the airport itself. When you do the search, than the name will be corrected to Brussel-Nationaal Luchthaven or just Brussel-Nat-Luchthaven.
The trains in belgium, except for a few special services, like thalys, don't have sit / train reservation. You can get any train.
The only detail that you must have in account is that to the airport you pay a an extra fee called "diabolo fee". If you buy the ticket in the machine or online it's usually included.