How to change the output strategy of a queue ?

Normally a queue releases the items on a first come first go basis. Is there a way that this decision can be taken on the basis of a label value on the item instead of first come first go.
Is there a possibility in Flexsim to add a dynamic label on an item. One whose value keeps on changing. I know its a discrete event based simulation package but still .. ??

No dynamic attributes. As you said, it is discrete event simulation so you have to find the event where the state of the items change one-by-one. Although in the real world an attribute may be constantly changing, there are usually certain thresholds that trigger a change in state. Also, sometimes this state change may only be observed upon an independent re-evaluation event that is periodic or scheduled.

You can try to sort the items as they enter the queue (OnEntry: Sort By Expression), however if the items are there to await their state change in order to leave then this will not suffice. Consider using a rack object as it has a dwell time function.

The approach with the most control is to manually release the items. Choose Do Not Release Item in the Send To Port of the queue (or just return -1), then it is up to you to determine when you want to release each item at a later time. A message trigger is usually used to hold evaluation logic that gets executed every time the state of an item changes, or when a downstream destination becomes available to receive again. It would have at least one for loop to evaluate all possible candidate items, using getitemstate() to make sure that you have not already released the candidate item (it could be waiting for a transport), then finally releaseitem(itemyouwanttorelease, portnum) once you have evaluated all candidates and found the best choice.