Refers to the many ways a speaker can refer to future time in English. There are five major ways and over a dozen ways total to refer to future time.
In English we don't conjugate a verb with future tense endings to refer to future time. Instead, we refer to future time in several different ways. These include using the auxiliary and modal will (Mary will leave Sunday or Mary will be leaving Sunday); the construction to be going to (Mary is going to leave Sunday); and other tense forms, including the present (Mary leaves Sunday), the present progressive (Mary is leaving Sunday); and and the past (Mary was leaving Sunday the last I heard). We can use all these ways, and more, to refer to future time. And each different way has its own subtle meaning. Thus the different ways of referring to the future time express differing attitudes of the speaker with regard to the future event or state, including a different expectation as to how likely the 'predicted' event will happen. English offers five major ways of referring to future time, but there are over a dozen ways altogether.