(because T6 is considered as a tramway line and not as a bus line, as the "T" shows)

In fact it was more about the type of vehicle, being very close in appearence to any type of tram (see https://fr.wikipedia.org/wiki/Ligne_6_du_tramway_d%27%C3%8Ele-de-France#/media/Fichier:57,900_T6_V%C3%A9lizy_Translohr_n%C2%B0621_par_Cramos.JPG), and very much different from what is shown in the English reference highway=bus_guideway. So I understood it as a specific kind of busway standard buses for example could take. The last part near Viroflay even looks like a classic subway line. Yet I must admit I've just seen that the French reference page shows exactly the same type of transportation than the T6 (i.e. a tram on wheels guided by a single rail): osm.wiki/FR:Tag:highway%3Dbus_guideway.
+ the way it is rendered in classic OSM style makes it appear very different from the rest of the tramway lines, which is a little counter-intuitive to me...