Eligibility depends on being passive, and qualified institutional investors file on a periodic schedule rather than within days. The result is that a 13G tells you an index fund or a large manager owns a big stake, which is usually mechanical rather than a view.
The signal lies in conversions. A holder moving from 13G to schedule-13d has abandoned passivity, and a new 13G appearing where an activist previously filed a 13D often means the campaign is over.
Example: a fund reports 7.2% on a 13G. Nine months later it files a 13D on the same stake, adding a purpose statement seeking board representation, and the stock reprices on the change of form alone.
Related: schedule-13d, form-13f, activist-investor, index-fund, voting-rights