A video is classified as viral when its plays exceed the creator's typical performance by a large multiple.
Viral posts are marked separately so they can be included or excluded when judging a creator's reliable baseline reach — for example when comparing median plays per post across shortlisted creators.
The concrete rule: a video counts as viral when its views exceed the median of the creator's other recent posts by a factor of 100, or when it was shown to a broad audience outside the follower base via the For You feed. The value counts how many of the creator's recent videos went viral: in the profile PDF export it is shown as "X of the last N", in the profile comparison as a percentage of the recent posts.
For campaigns, virality is helpful to reach far beyond the target audience. Keep in mind that viral videos are rare for smaller creators.