The orange alert applies to western and nothern parts of the country: Limburg province, Antwerp, East Flanders and West Flanders. Brussels and Flemish and Walloon Brabant "only" received a yellow warning.
The Met Office says the warnings may be adapted during the course of the day, as it reamins very tricky to predict thunderstorms, which can also be very local. An orange alert means that we may have 40 litres of rain (or more) in a short time, which can trigger local floods. A yellow alert is talking about "10 to 30 litres" per square metre, but this can also trigger local inundations.
The KMI says it's difficult to draw a line between yellow and amber. There is also a particular warning for hail balls, gusts of wind and a large frequency of lightning, which makes it a good idea to unplug your electrical appliances at home. Hail balls may - in a worst-case scenario - reach a size of 3 to 5 centimetres under an amber alert.