Hey gaboraa, there is also another option to your question. After you have all the faces selected on your mesh that you want to give that "certain" material goto Create > Sets > Quick Select Set. This will add all of those faces to this particular set, to go back and grab it just goto Edit > Quick Select Sets > then your set will be in there...it will be what ever you named it. As far as I know you can make as many sets as you want, but I have never made more than 3 so I am not sure.