@snarayanan , I suppose you were referring to FSM 6.3 (As you have mentioned IFS 6.3)? If so, there is no option given for backoffice as of now. But you can do so through a business rule configuration. Where you might need to create several validation business rules for each status for which you want to restrict status values as next option. 

However, I suppose the reason that option is not provided for Backoffice is, since the Backoffice personals should have the possibility to override the statuses when needed. E.g., In real time business scenario there can be times when technicians need to be shifted upon situations. Then, if the task is already accepted, technician do not have authority to reject an accepted task which needs to be controlled form back office (this is one example). Maybe it would be a good idea to drop a request on the RnD idea wall to seek if they have a future plan on introducing the same to back office. We can do that on behalf of you if you wish to.
 

Sorry for the delayed response. Thanks for the response. It would be great if you can put it on the RnD idea 

As of FSM6U5, Task Status Flow is supported and followed in the back office as well as in Mobile.

For any users to be able to update Task Status to a value not configured in the Task Status Flow, then the Function ‘TASKSTATUSANY’ (not disabled) needs to be added to their Role.