An ETF will have a creation and calculation basket. The creation basket represents the portfolio required to create more shares of the index. The calculation basket will include any “non-basket” securities. These are components that are still in the ETF but will no longer be part of any new shares. We need calculation basket since traders will hedge with this portfolio.
Reference:
http://www.indexuniverse.com/sections/features/9223-inside-the-nasdaq-100-rebalancing.html
PCF for ETF