If the note is because of StreetComplete asking you for the surface tag, then the answer is "paving stones".  We make no difference between this type of stones and the flat 30×30 paving stones commonly found on sidewalks.

(The relevant difference in Brussels is to know whether there is asphalt, paving stones or setts.  Those are the main 3 types you will see in the city, and the level of comfort is not the same, some routers use this info to put more weight on roads or paths that have a smooth surface.) 

BTW, didn’t we have this discussion already?
https://www.openstreetmap.org/note/3850026