Every index is an active strategy that has been written down. Eligibility screens set a universe (country, listing venue, minimum liquidity, free float), a weighting scheme distributes capital across it, and a schedule decides when the whole thing is refreshed.
Small rule differences produce large divergences. Two large-cap indices covering the same market can differ by several percent a year depending on how they treat dual listings, how quickly they admit new issues, and whether they apply buffer zones to reduce turnover.
Read the methodology document before treating an index as a neutral yardstick. The key sections are weighting (market-cap-weighted-index, equal-weighted-index), float treatment (free-float-adjustment) and review schedule (index-reconstitution).
Related: market-cap-weighted-index, equal-weighted-index, free-float-adjustment, index-reconstitution, index-fund, benchmark