I'm looking to fare conditions of Qatar airways ticket and see this
This ticket cannot be changed but can be reissued with a fee. What's the difference?
From a travel agent handbook which I managed to read many years ago, historically - in the era of paper tickets - there were two kinds of changes you can make:
A "change" was when you modified your schedule, but you kept and used the original ticket; the airline only registered the change in their system. Nowadays Delta calls this "standby". Most airlines were, and still are very restricted in doing changes (which legs, how, domestic/international etc). This kind of "change" for example happens if your first leg on a flight is delayed and as a result you would miss your connection - in this case the airline changes your whole itinerary (might route you through different airports), but not origin and destination. Your ticket number remains the same.
A "reissue" when you modified your schedule and got a new ticket with new details. There you have much less restrictions (essentially you cancel the old ticket, buy a new ticket, pay the fare difference and change fee).
In your case above the conditions are listed in case of a no-show. Thus the airline usually cancels automatically all remaining segments, so there's nothing to change. However the ticket remaining value still could be used until Dec 2018, and you can use it by "reissuing" the ticket - obviously for a different itinerary - once you pay the reissue fee.