This is true both when improvisation is to be performed in a choreographic situation in which specific limitations have been established and when the improvisation is completely ‘open’ and no preset decisions have been made. – Nalina Wait, pg. 140, in The Oxford Handbook of Improvisation in Dance.
If no preset decisions have been made, how would people know when and where to show up to do the improvised dance? In other words “open” is impossible.