This step can be used to grant:
Builds pass through the step in order (taking the build number as sorter field)
Older builds will not proceed (they are aborted) if a newer one already entered the milestone
When a build passes a milestone, any older build that passed the previous milestone - but not this one - is aborted.
Once a build passes the milestone, it will be never aborted by a newer build that didn't pass the milestone yet.