Guideline: Assign Changes to an Iteration
This guideline promotes the best practice of isolating team members from disruptive changes during the current iteration. Change requests are reviewed and prioritized during the current iteration, but are not acted upon until assigned to a future iteration.
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Main Description

Most iterative software development processes recommend that changes not be introduced during an iteration.  The main idea is that the iterations should be short and with clearly defined scope so that they can be time-boxed. 

To limit scope within an iteration, change requests are reviewed and prioritized as soon as possible, but are not assigned for implementation until a future iteration via the Artifact: Work Items List

Since iterations are relatively short this should not cause undue delay in dealing with urgent and important change requests.