Providers differ in method. Some run mechanical rules where the largest eligible companies enter automatically; others use a committee with discretion over timing and judgement calls such as profitability tests or dual-class-shares eligibility. The published methodology document is public and is the only reliable source for what actually qualifies a company.
Because index funds must hold what the provider says, a rule change is a real capital flow. Traders track methodology consultations the same way they track earnings.
Example: a provider announces that companies with non-voting share classes will be excluded at the next review. Funds tracking the index must sell those constituents, and the affected names underperform in the weeks before the effective date.
Related: index-inclusion, index-rebalance, index-reconstitution, free-float-weighting, gics