T - the callback result type. Depends on the function you're calling.
public interface Callback<T>
A callback. In some cases certain functions have to query the results from the
client first, so it's not possible to return the result immediately. For these
situations, you have to use callbacks: You can override the onCall()
method and put your code there. Once the server receives the results from the player,
the onCall() method of this callback will be called.
Every callback has a certain return type, i.e. an object that will be passed to
the onCall() method. Usually this is the actual result of the original