Here, in the declaration of the List[+T] type constructor (we can use the names parameterized types or type constructors interchangeably), we used the parameter name, T, it's a convention to use such names in generic programming. The names T,A,B, or C have nothing to do with the initializer type you're going to provide when you initiate a list instance. For example, when you give a String type for the previously mentioned type parameter when you instantiate List[String], it really doesn't matter if the declaration has List[T] or List[A]. What we mean is the following two declarations are equivalent:
//With type parameter name A sealed abstract class List[+A] extends AbstractSeq[A] //With type parameter name T sealed abstract class List[+T] extends AbstractSeq[T]