In Sevens Series terminology, "core teams" are those that are guaranteed a place in all series events in a given season. Unlike all other series events, the 2013 London Sevens had only 12 teams competing for series points, namely the top 12 core teams on the season points table following the  season's penultimate tournament, the Scotland Sevens.
In Sevens Series terminology, "core teams" are those that are guaranteed to participate in all series events during a season. The 2013 London Sevens was an exception because it only had 12 teams competing for points, and these were the top 12 core teams on the season standings after the Scotland Sevens tournament.