In interrogative sentences /che/ is the short form for /che cosa/, which is also a substitute for /quale cosa/ or /quali cose/ (these forms are rarely used). When /che/ is used to connect the primary to the secondary proposition, some of its usual meanings are /la quale cosa/, /le quali cose/. A detailed description of all usages of /che/ would turn out to be quite long, and out of my capabilities, as new problems arise along the way new questions (with some context) over the /che/ subject would certainly be welcomed here.