4.3.2.3 Skaters who are not part of the pack as illustrated in Figure 1 - In/Out of Play Example, but are still in play, may block and assist.

The engagement zone is mentioned quite a lot in the rules, and in rules explanations. The engagement zone is the area on the track where skaters are considered in play, and may legally engage opponents and assist teammates. It extends 20 feet from the front of the pack to 20 feet from the back of the pack. Since pack skaters must be in proximity, meaning within 10 feet of another pack skater, a skater may be in play while not in the pack. This illustration shows how that works.

http://wftda.com/rules/20100526/figure/1.jpg

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